Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
AIP Advances, 9, 5, pp. 1-8, 2019-05-10
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC :
https://nrc-publications.canada.ca/eng/view/object/?id=2276eef1-7877-41cc-8fbc-dda048f85f67
https://publications-cnrc.canada.ca/fra/voir/objet/?id=2276eef1-7877-41cc-8fbc-dda048f85f67
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.1063/1.5089658
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
Highly efficient p-type doping of GaN under nitrogen-rich and
low-temperature conditions by plasma-assisted molecular beam epitaxy
under nitrogen-rich and low-temperature
conditions by plasma-assisted molecular
beam epitaxy
Cite as: AIP Advances 9, 055008 (2019); https://doi.org/10.1063/1.5089658
Submitted: 21 January 2019 . Accepted: 01 May 2019 . Published Online: 10 May 2019 H. Tang, S. M. Sadaf, X. Wu, and W. Jiang
COLLECTIONS
Paper published as part of the special topic on Chemical Physics, Energy, Fluids and Plasmas, Materials Science and Mathematical Physics
ARTICLES YOU MAY BE INTERESTED IN
Heavy doping effects in Mg-doped GaN
Journal of Applied Physics
87, 1832 (2000);
https://doi.org/10.1063/1.372098
High hole mobility p-type GaN with low residual hydrogen concentration prepared by
pulsed sputtering
APL Materials
4, 086103 (2016);
https://doi.org/10.1063/1.4960485
High-voltage vertical GaN-on-GaN Schottky barrier diode using fluorine ion implantation
treatment
AIP Advances
ARTICLE scitation.org/journal/advHighly efficient p-type doping of GaN under
nitrogen-rich and low-temperature conditions
by plasma-assisted molecular beam epitaxy
Cite as: AIP Advances 9, 055008 (2019);doi: 10.1063/1.5089658Submitted: 21 January 2019 • Accepted: 1 May 2019 • Published Online: 10 May 2019
H. Tang,a) S. M. Sadaf, X. Wu, and W. Jiang
AFFILIATIONS
National Research Council, Ottawa, Ontario K1A 0R6, Canada
a)Email:Haipeng.Tang@nrc.ca
ABSTRACT
Highly efficient and reproducible p-type doping of GaN under nitrogen-rich and low-growth-temperature conditions was demonstrated with the plasma-assisted molecular beam epitaxy technique. The low-temperature range is approximately below 650○C and refers to growth temperatures at which the thermal desorption of any excess Ga is negligibly slow. The Mg and hole concentrations obtained with the N-rich condition were more than one order of magnitude higher than those obtained with the Ga-rich condition while keeping all other conditions identical. The Mg doping under such N-rich conditions was also found to show Mg-mediated suppression of background impurities, good epitaxy quality on GaN templates, and relatively low surface roughness. Over the investigated growth temperature range from 580○C to 650○C, the Mg incorporation efficiency under the N-rich condition was found to be close to unity (70%-80%) and independent of the growth temperature. High hole concentrations of up to 2×1019cm-3and activation efficiencies of up to 16.6% were obtained. The result rules out the Mg surface sticking probability as the limiting mechanism for Mg incorporation in this temperature range, as it would be temperature dependent. Instead, the Mg incorporation rate was more likely governed by the availability of substitutional sites for Mg on the surface, which should be abundant under the N-rich growth conditions. Excellent diode characteristics and electroluminescence results were observed when this p-type doping method was employed in the growth of full device structures.
© 2019 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).https://doi.org/10.1063/1.5089658
I. INTRODUCTION
The efficient p-type doping in plasma-assisted molecular beam epitaxy (PA-MBE) makes this growth technique a viable alter-native to the metal-organic vapor phase epitaxy technique for a wide range of optoelectronic devices including lasers.1,2In addition, the Mg doping by MBE does not require post-growth activation, which is unique advantage for growth of tunnel-junction devices.3–8 Although successful p-type doping has been reported for the plasma MBE method under various conditions, the delineation and under-standing of the different growth regimes is still not definitive or com-plete.9The III/V ratio has profound impact on the growth by plasma MBE. The growth temperature range can be generally divided into the “low temperature” and “high temperature” growth regimes with the demarcation roughly at 650 ○C. Under slight or moderate Ga-rich (III > V) conditions, the excess Ga forms a self-stabilized
Ga bilayer in the low temperature regime, but not in the high tem-perature regime. This is because the excess Ga adatoms are rapidly desorbed in the high temperature regime. In the growth of undoped GaN, it has been well established that the Ga-rich and Ga bilayer growth condition is critically important for high epitaxy quality especially in the low temperature growth regime.10Correspondingly, the majority of previous studies on p-type doping with PA-MBE indeed concentrated on the Ga-rich growth conditions, either in the low-temperature range (<650○C),11–15or in the high-temperature range of 670○C to 780○C.16–19
To date, the p-type doping using N-rich conditions has been much less investigated and remained inconclusive, especially in the low-growth-temperature regime. There have been inconsistent results in the literature regarding the relation between the III/V ratio and the Mg incorporation efficiency. Some report suggested that Ga-rich conditions enhanced Mg incorporation,20 while another
AIP Advances 9, 055008 (2019); doi: 10.1063/1.5089658 9, 055008-1
study associated N-rich conditions to polarity flipping and thus poor Mg doping efficiency.21In other studies, Mg doping under N-rich conditions and at low temperatures yielded highly resistive films.11,15 Successful p-type doping under N-rich conditions was only reported at high growth temperatures (around 740○C).22However, in such high-temperature regime, the effect of the III/V ratio is less pro-found, because under both N-rich and Ga-rich conditions, the Ga bilayer could not form due to much accelerated thermal desorp-tion. The N-rich condition was first explicitly suggested to be favor-able or necessary for efficient Mg incorporation at low growth tem-peratures by a series of studies of p-type doping using the metal-modulated-epitaxy (MME) technique.23–26In the MME growth, the Ga flux is set to be greater than the nitrogen flux, and is periodically shuttered while maintaining the nitrogen flux constant. During the growth, metallic buildup occurs under Ga-rich conditions but deple-tion of both the metallic Ga bilayers and the surface-accumulated Mg dopants occur during the transition from Ga-rich to N-rich conditions.23 Such an “oscillation” of growth conditions can be achieved by periodic modulation of the metal fluxes while maintain-ing a constant nitrogen flux.23It was suggested that the Mg incor-poration rate was much higher during the N-rich period than the Ga-rich period with the latter only serving to restore the crystal qual-ity and surface smoothness. The overall Mg doping concentration increased when the Ga-rich period was shortened relative to the N-rich period.23–26 However, when the Ga-rich period was made much shorter than the N-rich period, the p-type layer quality became worse and the layer turned highly resistive.24 To date, the MME studies have not reported p-type doping under constant N-rich conditions.
In the present study, highly efficient and reproducible p-type doping under constant N-rich and low-growth-temperature condi-tions was observed and confirmed through detailed characterization of a series of comparative samples. Comparisons were made with Ga-rich grown samples as well as undoped samples. Our detailed study discovered that under the N-rich conditions, the Mg dopant mediated suppression of background oxygen and carbon impuri-ties, leading to excellent low-compensation p-type materials. Excel-lent device characteristics were obtained when this p-type doping method was applied in the growth of full device structures. II. EXPERIMENTAL
All the growths were performed using an SVTA nitride MBE system equipped with a Veeco unibulb plasma source. Meticulous care was taken to ensure the highest system and effusion source purity. All substrates were HVPE Fe-doped semi-insulating GaN on sapphire templates with 5 µm thickness and dislocation density in the order of 1×108cm-2. The substrates were dipped in HF solution to remove surface oxide prior to loading for growth. The surface was further cleaned in situ by Ga adsorption and desorption treatment at around 700○C. The growth temperature was calibrated with mea-surements of the characteristic Ga surface desorption time constant. The nitrogen flux was fixed at 220 W plasma power and 1.3 sccm N2 flow which yielded equivalent nitrogen-limited growth rate of 0.36 µm/hr. The III/V ratio was set at 0.8 : 1 for the N-rich growth and 1.1 : 1 and for the Ga-rich growth. Two moderate/low Mg dopant flux with beam equivalent pressure (BEP) of 4×10-10Torr (Mg cell at 310○C) and 2×10-10Torr (Mg cell at 290○C) were selected for
the doping experiments to ensure absence of polarity flipping. The growth duration was kept at 1.5 hours, yielding thickness of 0.45 µm for the N-rich growth, and 0.55 µm for the Ga-rich growth.
The secondary ion mass spectrometry (SIMS) measurements were performed by Evans Analytical Group (EAG Laboratories). Hall-effect and resistivity measurements were performed using an Accent HL5500 Hall system. The measurements were carried out at room temperature on 1.5 cm × 1.5 cm size samples using the van der Pauw method and square geometry with the ohmic contacts formed on the four corners.
III. RESULTS AND DISCUSSION
Figure 1 shows the SIMS profiles of Mg-doped GaN layers grown at 580○C and Mg flux of 4×10-10Torr under different III/V ratio conditions.Fig. 1(a)and(b)compare the doping profiles under constant N-rich or Ga-rich conditions. Constant Mg profiles were observed in both samples. The Mg concentration was at 1.2×1020 cm-3in the N-rich sample, which is about 20 times higher than the value of 6∼7×1018 cm-3 in the Ga-rich sample. This result shows that the Mg incorporation efficiency is dramatically higher under the N-rich condition than under the Ga-rich condition with all other conditions kept identical.
It is worth mentioning that the Mg/Ga flux ratio is somewhat higher for the N-rich growth which may partially contribute to the higher Mg concentration. The Ga/N flux ratio was 1.1 for the Ga rich growth and 0.8 for the N-rich growth conditions. The Mg flux (BEP) was kept at 4×10-10Torr. The Ga flux (BEP) was 2.2×10-7and 3.0×10-7Torr for the N-rich and Ga-rich growth runs, respectively. The Mg/Ga flux ratio was calculated according to:27
JMg/JGa= (PMgηGa/PGaηMg) √
MGaTMg/MMgTGa (1) where PGaand PMgare beam equivalent pressures, ηGaand ηMg ion-ization efficiencies, MGaand MMgmolar masses, and TGaand TMg effusion cell temperatures. An estimate of the ionization efficiency is given by:27
η/η(N2) = [(0.4Z/14) + 0.6] (2)
where Z is the atomic number of the measured flux species. The Mg/Ga flux ratio was calculated to be 0.35% and 0.26% for the N-rich growth and Ga-rich growth, respectively. Thus the Mg/Ga flux ratio was about 25% higher in the N-rich growth than in the Ga-rich growth, which may contribute to some but not all of the 20 times increase of Mg incorporation in the N-rich growth.
The sample inFig. 1(c)was grown under Ga-rich condition except the Ga flux was interrupted for 12 s after every 2 minutes of growth. This Ga interruption (with nitrogen plasma always on) would momentarily remove excess Ga and thus prevent buildup of a full Ga bilayer or Ga droplets. No Ga droplets were found on the surface after growth. The Mg doping profile in this sample shows non-constant Mg concentration varying between 1.2×1019cm-3and 8×1019cm-3. This shows sensitive dependence of the Mg incorpora-tion rate on the surface coverage of excess Ga which was modulated by the Ga flux interruptions in this sample. Thus, the Ga bilayer and droplets formed under the Ga-rich condition appear to be a severe barrier for Mg lattice incorporation.
In the nitrogen rich p-type sample, the background oxygen level was approximately at 3×1017cm-3and the oxygen spike at the
AIP Advances
ARTICLE scitation.org/journal/advFIG. 1. SIMS depth profiles for Mg doped GaN layers grown under (a) nitrogen
rich condition (III/V = 0.8: 1), (b) Ga rich condition (III/V = 1.1: 1), and (c) Ga rich condition (III/V=1.1:1) but with Ga flux interruption. The growth temperature was all at 580○C, and the Mg beam equivalent pressure at 4×10-10Torr.
substrate interface was quickly suppressed as shown in Fig. 1(a). In the Ga rich p-type sample, the interface oxygen spike extended into the epilayer. The Ga-rich sample was covered with Ga droplets during growth which were removed by HCl etching after growth. However, the etching of the Ga droplets left behind residue marks on the surface, which could be seen under optical microscope. The existence of such residue marks on the surface of the Ga-rich sam-ple may explain the pronounced influence of surface artifacts in the SIMS profiles inFig. 1(b). Due to significant extension of the surface and interface artifacts in this sample, the true background impu-rity concentrations can’t be accurately established. However, they should be somewhat below the lowest point of the measured pro-filing curves. For example, the lowest point of the oxygen profile is at 8×1017 cm-3. This value may be regarded as the upper limit for the estimate of the true background oxygen concentration. Ideally, thicker layers are required to determine the background impurity concentrations accurately.
The electrical properties of the nitrogen rich p-type sample in
Fig. 1(a)and the Ga rich p-type sample inFig. 1(b)are presented in
Table Itogether with data on Mg/Ga flux ratio, Mg incorporation efficiency, and hole activation efficiency. As previously described, the Mg/Ga flux ratio was calculated according to equations(1)and
(2). The Mg incorporation efficiency (defined as the Mg incorpora-tion/Mg dose ratio) was calculated based on the measured Mg den-sity in the film and the calculated Mg/Ga flux ratio. The activation efficiency is defined as the hole density/Mg density ratio.
Thus the N-rich condition resulted in not only significantly higher Mg concentration but also much higher concentration of activated holes. The Mg incorporation efficiency was 80% for the N-rich condition versus 5% for the Ga-rich condition. The hole concentration was 2 × 1019 cm-3 for the N-rich condition versus 7.5 × 1017 cm-3 for the Ga-rich condition. Both samples showed quite high activation efficiencies.
The p-type doping under N-rich conditions was scalable to lower doping levels by reducing the Mg flux. It was also effective for a wide range of growth temperatures.Table IIgives a summary of the p-type doping results for various Mg flux and growth temperatures under the N-rich conditions.
The Mg concentrations are found to be roughly proportional to the Mg flux (BEP). The Mg incorporation efficiency was found to be almost constant with growth temperature changing from 580○C to 650○C. Within the accuracy of the calculated Mg/Ga flux ratios, the Mg incorporation efficiency is found to be very close to unity at 70% - 80% levels. This indicates that the limiting factor for Mg incorporation in this case is not the often suggested Mg surface stick-ing probability, as the latter is temperature dependent. Instead, it is more likely governed by the availability of substitutional sites for Mg on the growth surface, which would be abundant under the N-rich growth conditions.
Quite high activation efficiencies ranging from 6.3 % to 16.6% were found in the samples inTable II. The high activation efficiency of 16.6% at high Mg doping density of 1.2×1020cm-3is similar to the results by Fischer et al.25High activation efficiencies were com-monly observed when the Mg doping concentration approaches the degeneracy limit.23–26
The authors initially hesitated to explore the N-rich condi-tions for p-type doping due to the well-known phenomenon that
AIP Advances 9, 055008 (2019); doi: 10.1063/1.5089658 9, 055008-3
TABLE I. P-type doping results under N-rich and Ga-rich conditions at growth temperature of 580○C.
Mg BEP Mg/Ga Flux Mg density Hole density Mobility Resistivity
Ga/N Flux ratio (Torr) ratio (cm-3) (cm-3) (cm2/Vs) (Ω cm) Mg incorp. efficiency Activation efficiency
0.8 4 × 10-10 0.35% 1.2 × 1020 2 × 1019 0.7 0.43 80% 16.6%
1.1 4 × 10-10 0.26% 7 × 1018 7.5 × 1017 4.7 1.90 5% 10.7%
TABLE II. P-type doping results under N-rich conditions for different Mg flux and growth temperatures.
Mg BEP Mg density Hole density Mobility Resistivity
T (growth) (Torr) Mg/Ga Flux ratio (cm-3) (cm-3) (cm2/Vs) (Ω cm) Mg incorp. efficiency Activation efficiency
580○C 4 × 10-10 0.35% 1.2 × 1020 2 × 1019 0.7 0.43 80% 16.6%
580○C 2 × 10-10 0.17% 5.4 × 1019 3.4 × 1018 2.3 0.80 72% 6.3%
620○C 2 × 10-10 0.17% 5.5 × 1019 3.9 × 1018 1.0 1.5 73% 7.1%
650○C 2 × 10-10 0.17% 5.3 × 1019 4.5 × 1018 1.0 1.3 71% 8.5%
the N-rich conditions would normally introduce large amount of background impurities in undoped GaN epilayers especially at low temperatures.28–30It was found to be true also in our growth sys-tem for the undoped GaN layers as shown by the SIMS profiles in
Fig. 2. Changing the growth condition from Ga-rich to N- rich at the growth temperature of 580○C caused the oxygen, carbon and hydro-gen concentrations to rise by more than one order of magnitude in the un-intentionally doped GaN. In particular, the donor-type oxygen impurity reached formidable high 1018cm-3levels.
The surface artifacts are less pronounced in the SIMS profiles of the Ga-rich, undoped GaN sample inFig. 2(b)than the Ga-rich, Mg-doped GaN sample inFig. 1(b). This is because the etching residues from Ga droplets were less prevalent on the undoped GaN samples than on the Mg-doped GaN samples. The relatively flat regions of the depth profiles give an estimate of the oxygen concentration at 2×1017cm-3and carbon concentration at about 5×1015cm-3.
In contrast, the undoped GaN grown under the N-rich condi-tions exhibited more than an order-of-magnitude higher oxygen as well as carbon concentration, as shown inFig. 2(a). From the inter-face towards the surinter-face, the oxygen concentration increases from 2×1018to 1×1019cm-3, and carbon from 2×1017to 8×1018cm-3. The high background impurity concentrations in this sample are unlikely artifacts caused by surface roughness. The surface artifacts are much less pronounced in the nitrogen rich samples since the surface is free from any residues and the RMS roughness is minor (<5nm). This conclusion can be inferred by comparison of the SIMS profiles of the two samples inFigs. 1(a)and2(a), which were both grown under N-rich conditions and had the same degree of surface roughness. In
Fig. 1(a), the surface artifacts are clearly insignificant and restricted in the surface region. Therefore, the high impurity concentrations inFig. 2(a)are considered to be true results rather than artifacts caused by roughness. Instead, such nano-scale roughness could have induced actual high impurity incorporation during the growth of the undoped GaN sample. The evolving SIMS profiles along the film thickness inFig. 2(a)can be reflection of increasing impurity incor-poration as the surface roughness evolves with the growth thickness. Of course, thicker layers would be more ideal for the analysis as they may allow the impurity profiles to reach saturated flat regions.
FIG. 2. SIMS depth profiles for undoped GaN layers grown under (a) nitrogen rich
condition (III/V = 0.8: 1), and (b) Ga rich condition (III/V = 1.1: 1). The growth temperature was at 580○C.
AIP Advances
ARTICLE scitation.org/journal/advFIG. 3. TEM cross-sectional images of Mg doped GaN
layers grown on semi-insulating GaN templates under (a) nitrogen rich condition (III/V = 0.8: 1), and (b) Ga rich con-dition (III/V = 1.1: 1) at growth temperature of 580○C, and
Mg beam equivalent pressure of 4×10-10Torr. The growth
was in the Ga-polar [0001] direction with the open arrows marking the position of the interface with the substrate. The images were taken at [11-20] zone axis.
The undoped sample grown under the Ga-rich conditions showed resistivity of 0.11 Ωcm, electron concentration of 3.6 × 1017 cm-3, and mobility of 156 cm2/Vs. On the other hand, the undoped sample grown under the N-rich conditions was found to be highly resistive. From the SIMS profiles in Fig. 2(a), both the oxygen and carbon concentrations increased to the 1018 cm-3 levels. The high resistivity of this sample is likely a result of compensation by acceptor-type carbon impurities. The interplay between oxygen and carbon impurities was also observed in our previous work in Ref. 31. The Fig. 7 in Ref. 31shows rapid drop in the electron concentration in undoped GaN layers grown by plasma-assisted MBE when the carbon concentration approaches the oxygen con-centration despite of high oxygen concon-centration in the order of 1018 cm-3. It is known that carbon doping can result in semi-insulating GaN in MBE growth.32It is worth pointing out that under similar N-rich growth conditions, highly conducting undoped GaN films were obtained with carrier density of 1.8×1019 cm-3, oxygen con-centration of 4×1019cm-3, and carbon concentration of 2×1018cm-3 in our earlier work back in 2004.31In comparison, the highly resis-tive sample in the present study shows about 10 times lower oxygen concentration but same level of carbon concentration. This sug-gests that the system purity (gas, solid sources, vacuum) was sig-nificantly improved in the present work than in the earlier work in 2004.
The results on the undoped GaN layers cast real doubt whether p-type conductivity could ever be achieved if background impurities
of such high concentrations are present. However, the present study discovered that with the addition of Mg dopants, the background impurities (O, C and H) were remarkably suppressed under the nitrogen-rich and low-temperature conditions. This Mg-mediated effect is quite vividly illustrated by the comparison of the impurity profiles inFig. 1(a)andFig. 2(a). The Mg adatoms on the growth surface could react with impurities and form volatile species. The Mg mediated suppression of background impurities thus enabled p-type doping with low compensation even under the low-temperature and nitrogen-rich conditions. On the other hand, under Ga-rich condi-tions, the Mg dopant did not show effect of impurity suppression, as illustrated by the comparison betweenFig. 1(b)andFig. 2(b). In this case, both the Mg adatoms and the background impurities are dissolved into the Ga bilayer. The impurity suppression is primarily mediated by the Ga bilayer rather than by the Mg adatoms.
For hetero-epitaxy, the nitrogen-rich and low-temperature conditions generally produce poor crystal quality and rough surface morphology.10,24,33 However, in the present work, the p-type lay-ers were grown via homo-epitaxy on high quality GaN templates, and were found to have preserved the high crystalline quality of the templates to a large degree.Fig. 3(a)and(b)shows the TEM cross-sectional images of the N-rich sample inFig. 1(a)and the Ga-rich sample inFig. 1(b)respectively.
In both samples, the threading dislocations from the GaN template extended into the p-type layers. In the N-rich sample, no new dislocations were generated from the substrate interface.
FIG. 4. AFM images of Mg doped GaN layers grown (a) at 580○C under nitrogen rich condition (III/V = 0.8: 1), (b) at 580○C under Ga rich condition (III/V = 1.1: 1), and (c)
at 740○C under Ga rich condition (III/V = 1.1: 1).
AIP Advances 9, 055008 (2019); doi: 10.1063/1.5089658 9, 055008-5
Instead, some stacking faults (cubic inclusions) were formed at or away from the interface. Stacking faults are found to be absent in the Ga-rich sample. However, there seem to be additional disloca-tions being generated at the substrate interface as shown inFig. 3(b). In particular, the N-rich sample showed better epitaxy quality at the interface showing no visible boundary with the substrate template in the TEM image. The better interface quality could be due to the Mg mediated suppression of the interface contaminants which is more effective with the N-rich condition. The SIMS impurity profiles in
Fig. 1(a)and(b)also showed less pronounced interface spikes of contaminants in the nitrogen rich p-type sample than in the Ga rich p-type sample.
From the TEM images inFig. 3, the Ga-rich sample shows per-fectly smooth surface while the N-rich sample shows certain rough-ness.Fig. 4(a)and(b)show the AFM images of p-type GaN layers grown at 580○C under N-rich and Ga-rich conditions, respectively. For comparison, the AFM morphology of a p-type GaN layer grown at high temperature (740○C) is shown inFig. 4(c). As expected, the Ga-rich sample grown at 580○C shows atomic steps due to layer-by-layer growth mode enabled by the Ga bilayer-by-layer, with RMS roughness less than 1 nm. The N-rich sample grown at 580○C showed RMS roughness of 4.5 nm. At this roughness level, the film was optically clear and showed no appreciable light scattering. The high temper-ature grown p-type sample, on the other hand, showed significantly higher roughness with RMS roughness typically around 20 nm. The film showed marked optical haziness due to light scattering. P-type doping at high growth temperatures has been investigated in another submitted work and in the literature.16–19
The p-type doping under nitrogen-rich and low-temperature growth conditions has also been applied to the effort to grow InGaN/GaN quantum well lasers by plasma-assisted MBE in our laboratory. The separate-confinement quantum-well laser structure was grown on 10.5 mm × 10 mm size n-type HVPE grown bulk GaN substrates. A schematic diagram of the laser stack is given in Fig. 5(a). The wafers were fabricated into 20 µm wide broad-area metal stripe structures without mesa etching or oxide isolation using standard photolithography and metal deposition techniques. The p-type metal stripe was made of 5 nm Ni/50 nm Au annealed at 500○C followed by 5 nm Ni/500 nm Au. The wafers were then cleaved to form 20 µm × 1 mm bars for electrical testing. The n-type contact was formed by indium bonding of the back side of the n-type GaN substrate to metal heat sink.
Figure 5(b)shows the current-voltage (I-V) characteristics of such a laser structure where the p-type region (p-GaN:[Mg] 1×1019 cm-3, p-AlGaN:[Mg] 5×1019cm-3, and p++-GaN:[Mg] 1×1020cm-3) was grown under the nitrogen-rich and low-temperature conditions as described above. The I-V characteristics were measured with an HP4155 parameter analyzer under continuous wave (CW) biasing condition. The reverse current was measured up to -10 V, and for-ward current measured up to the setup compliance of 100 mA. The I-V curve shows excellent diode characteristics with a sharp turn on at voltage of ∼2.8 V and very low reverse leakage current up to -10 V. We have compared the different p-type doping regimes by their influence on the diode characteristics of the devices. The low-temperature, nitrogen-rich doping condition consistently resulted in the lowest reverse leakage current.
Figure 5(c)shows the spontaneous-emission electrolumines-cence (EL) spectrum under CW biasing condition of the same device
FIG. 5. (a) Schematic diagram of a separate-confinement InGaN/GaN
quantum-well laser structure where the p-type region was grown under the low temperature and nitrogen rich conditions; (b) I-V characteristics of this device structure fabri-cated to 20µm × 1 mm broad area bars; (c) spontaneous electroluminescence spectrum and photo from the device bar.
as inFig. 5(b). The inset shows the optical image of the device under an injection current of 20 mA (100 A/cm2). It shows a relatively nar-row and single emission peak at 410 nm. The spectral line width (full width at half maximum) is ∼22 nm. Although intense spon-taneous emission in milli watt range was obtained when the devices were pumped to higher current densities, we have not yet achieved device lasing so far, regardless of what doping methods we used for the p-type region. In order to demonstrate lasing devices, we are cur-rently working on improving the overall quality of the laser stack, the
AIP Advances
ARTICLE scitation.org/journal/advefficiency of the multiple quantum wells, and the optical confine-ment of the waveguide structure.
Although p-type doping under Ga-rich conditions yielded the smoothest surface morphology (RMS roughness ≤ 1nm), it was less efficient and required real-time Ga flux control in order to pre-vent formation of Ga droplets which could lead to device shorting and leaking. Such active modulation of the Ga flux during growth often caused irreproducibility and variation of the doping results, as illustrated by the varying SIMS profile inFig. 1(c). P-type dop-ing at high growth temperature was found to be quite efficient and reproducible. However, it generally resulted in high-roughness sur-face with RMS roughness typically around 20-25 nm. In compari-son, the p-type GaN and AlGaN layers grown under the N-rich and low-temperature conditions typically showed relatively small RMS roughness of 2 – 5 nm. The total integrated scattering (TIS) of light by a surface is given by:34
TIS(Rq) % = R0 ⎡⎢ ⎢⎢ ⎣1 − e −(4πRqcosθ λ ) 2⎤⎥ ⎥⎥ ⎦ (3)
where Rq is the RMS roughness (nm), R0 the theoretical surface reflectance, λ the light wavelength (nm), and θ the light inci-dence angle. For normal inciinci-dence of 450 nm wavelength light and assuming R0=18% for GaN surface, the scattering loss was calcu-lated to be 0.015%, 0.34%, and 6.8% for Rq = 1nm, 5 nm, and 25 nm, respectively. By visual inspection, the p-type layers grown under both low-temperature/Ga-rich and low-temperature/N-rich conditions are optically clear, whereas the p-type layers grown under the high-temperature/Ga-rich conditions show pronounced hazy appearance. We expect to investigate further the effect of the different degrees of surface roughness on the operation of laser devices once we achieve lasing devices in the future. On the other hand, the moderate surface roughness would not be a concern for LEDs devices as it may enhance light extraction through roughness scattering.
IV. CONCLUSIONS
In conclusion, p-type doping of GaN under N-rich and low-temperature conditions was found to be highly efficient and repro-ducible. When other conditions were identical, the N-rich condi-tion yielded more than one order-of-magnitude higher p-type dop-ing concentrations than the Ga-rich condition. The p-type dopdop-ing under such conditions showed Mg-mediated suppression of back-ground impurities, good epitaxy quality on GaN templates, and relatively low surface roughness. Over the investigated growth tem-perature range from 580○C to 650○C, the Mg incorporation effi-ciency under the N-rich conditions was found to be close to unity (70%-80%) and independent of the growth temperature. High hole concentrations of up to 2×1019cm-3and activation efficiency of up to 16.6% were obtained. Excellent diode characteristics and elec-troluminescence results were observed when this p-type doping method was applied in the growth of full structures of light-emitting devices.
ACKNOWLEDGMENTS
The authors wish to thank Simona Moisa for the AFM char-acterization, Ryszard Dabkowski for technical assistance with MBE
growth, and Kelly laliberte and Frances Lin for assistance with the device fabrication.
REFERENCES
1
C. Skierbiszewski, H. Turski, G. Muziol, M. Siekacz, M. Sawicka, G. Cywinski, Z. R. Wasilewski, and S. Porowski,J. Phys. D: Appl. Phys.47, 073001 (2014).
2
M. Zhang, A. Banerjee, C.-S. Lee, J. M. Hinckley, and P. Bhattacharya,Appl. Phys. Lett.98, 221104 (2011).
3F. Akyol, S. Krishnamoorthy, Y. Zhang, J. Johnson, J. Hwang, and S. Rajan,Appl.
Phys. Lett.108(13), 131103 (2016).
4
A. I. Alhassan, E. C. Young, A. Y. Alyamani, A. Albadri, S. Nakamura, S. P. DenBaars, and J. S. Speck, Appl. Phys. Exp. 11(4) (2018).
5
E. A. Clinton, E. Vadiee, S.-C. Shen, K. Mehta, P. Douglas Yoder, and W. Alan Doolittle,Appl. Phys. Lett.112(25), 252103 (2018).
6M. Malinverni, D. Martin, and N. Grandjean,Appl. Phys. Lett.
107(5), 051107 (2015).
7
S. M. Sadaf, Y. H. Ra, S. Zhao, T. Szkopek, and Z. Mi,Nanoscale11(9), 3888 (2019).
8
C. Skierbiszewski, G. Muziol, K. Nowakowski-Szkudlarek, H. Turski, M. Siekacz, A. Feduniewicz-Zmuda, A. Nowakowska-Szkudlarek, M. Sawicka, and P. Perlin, Appl. Phys. Exp. 11(3) (2018).
9
Y.-H. Liang and E. Towe,Appl. Phys. Rev.5, 011107 (2018).
10
B. Heying, R. Averbeck, L. F. Chen, E. Haus, H. Riechert, and J. S. Speck,J. Appl. Phys.88, 1855 (2000).
11
E. Haus, I. P. Smorchkova, B. Heying, P. Fini, C. Poblenz, T. Mates, U. K. Mishra, and J. S. Speck,J. Crystal Growth246, 55 (2002).
12A. Feduniewicz, C. Skierbiszewski, M. Siekacz, Z. R. Wasilewski, I. Sproule,
S. Grzanka, R. Jakiela, J. Borysiuk, G. Kamler, E. Litwin-Staszewska, R. Czernecki, M. Bo´ckowski, and S. Porowski,J. Crystal Growth278, 443 (2005).
13I. P. Smorchkova, E. Haus, B. Heying, P. Kozodoy, P. Fini, J. P. Ibbetson,
S. Keller, S. P. DenBaars, J. S. Speck, and U. K. Mishra,Appl. Phys. Lett.76, 718 (2000).
14
A. J. Ptak, T. H. Myers, L. T. Romano, C. G. Van de Walle, and J. E. Northrup, Appl. Phys. Lett.78, 285 (2001).
15J. Simon and D. Jena,Phys. Stat. Sol. (a)
205, 1074 (2008).
16W. C. Yang, P. Y. Lee, H. Y. Tseng, C. W. Lin, Y. T. Tseng, and K. Y. Cheng,
J. Crystal Growth439, 87 (2016).
17
A. Bhattacharyya, W. Li, J. Cabalu, T. D. Moustakas, D. J. Smith, and R. L. Hervig,Appl. Phys. Lett.85, 4956 (2004).
18
R. R. Lieten, V. Motsnyi, L. Zhang, K. Cheng, M. Leys, S. Degroote, G. Buchowicz, O. Dubon, and G. Borghs,J. Phys. D: Appl. Phys.44, 135406 (2011).
19G. Koblmüller, F. Wu, T. Mates, J. S. Speck, S. Fernández-Garrido, and
E. Calleja,Appl. Phys. Lett.91, 221905 (2007).
20
A. Bhattacharya, W. Li, J. Cabalu, T. D. Moustakas, D. J. Smith, and R. L. Hervig, Appl. Phys. Lett.85, 4956 (2004).
21
D. S. Green, E. Haus, F. Wu, L. Chen, U. K. Mishra, and J. S. Speck,J. Vac. Sci. Technol. B21, 1804 (2003).
22M. Zhang, P. Bhattacharya, W. Guo, and A. Banerjee,Appl. Phys. Lett.
96, 132103 (2010).
23
N. Gon, E. Trybus, K. K. Lee, M. Moseley, W. Alan Doolittle, and D. C. Look, Appl. Phys. Lett.93, 172112 (2008).
24
E. Trybus, W. A. Doolittle, M. Moseley, W. Henderson, D. Billingsley, N. Gon, and D. C. Look,Phys. Status Solidi C6, S789 (2009).
25A. M. Fischer, S. Wang, F. A. Ponce, B. P. Gunning, C. A. M. Fabien, and
W. A. Doolittle,Phys. Status Solidi B254, 1600668 (2017).
26
B. P. Gunning, C. A. M. Fabien, J. J. Merola, E. A. Clinton, W. Alan Doolittle, S. Wang, A. M. Fischer, and F. A. Ponce,J. Appl. Phys.117, 045710 (2015).
27
R. A. Kubiak, S. M. Newstead, and P. Sullivan, “The technology and design of molecular beam epitaxy systems,” in Molecular Beam Epitaxy, R. F. C. Farrow, editor, New Jersey, 1995, p. 1–113.
28
C. R. Elsass, T. Mates, B. Heying, C. Poblenz, P. Fini, P. M. Petroff, S. P. DenBaars, and J. S. Speck,Appl. Phys. Lett.77, 3167 (2000).
29C. Poblenz, T. Mates, M. Craven, S. P. DenBaars, and J. S. Speck,Appl. Phys.
Lett.81, 2767 (2002).
AIP Advances 9, 055008 (2019); doi: 10.1063/1.5089658 9, 055008-7
30
A. J. Ptak, L. J. Holbert, L. Ting, C. H. Swartz, M. Moldovan, N. C. Giles, T. H. Myers, P. Van Lierde, C. Tian, R. A. Hockett, S. Mitha, A. E. Wickenden, D. D. Koleske, and R. L. Henry,Appl. Phys. Lett.79, 2740 (2001).
31
H. Tang, S. Rolfe, M. Beaulieu, I. Sproule, S. Haffouz, and J. Webb, ECS Proc
2004-6, 215 (2004).
32
H. Tang, J. B. Webb, J. A. Bardwell, S. Raymond, S. Joseph, and C. Uzan-Saguy, Appl. Phys. Lett.78, 757 (2001).
33
C. D. Lee, A. Sagar, R. M. Feenstra, C. K. Inoki, T. S. Kuan, W. L. Sarney, and L. Salamanca-Riba,Appl. Phys. Lett.79, 3428 (2001).
34
