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MULTIQUANTUM WELL GaAs/AlGaAs STRUCTURES APPLIED TO AVALANCHE
TRANSIT TIME DEVICES
D. Lippens, O. Vanbesien, B. Lambert
To cite this version:
D. Lippens, O. Vanbesien, B. Lambert. MULTIQUANTUM WELL GaAs/AlGaAs STRUCTURES APPLIED TO AVALANCHE TRANSIT TIME DEVICES. Journal de Physique Colloques, 1987, 48 (C5), pp.C5-487-C5-490. �10.1051/jphyscol:19875103�. �jpa-00226685�
Colloque C5, suppl6ment au noll, Tome 48, novembre 1987
MULTIQUANTUM WELL GaAs/AlGaAs STRUCTURES APPLIED TO AVALANCHE TRANSIT TIME DEVICES
D. LIPPENS, 0. VANBESIEN and B. LAMBERT"
Centre Hyperfrequences et Semi-Conducteurs, UA 287, CNRS, Universitb des Sciences et Techniques de Lille-Flandres-Artois, F-59655 Villeneuve-d'Ascq Cedex, France
*centre National dlEtudes des TelBcommunications, Lab. ICM, F-22301 Lannion Cedex, France
Rkurnk Nous avons effectue des simulations particulaires de l'ionisation par choc dans des h6tirostructures GaAsIGaAIAS afin d'haluer les performances potentielles B 100 GHz d'oscillateurs B avalanche et temps de transit multicouches. Nous montrons sur deux exemples de structures i modulation de semiconducteurs qu'il est possible de controler et d'assister le processus d'imission de porteurs par ionisation par choc. Ceci se traduit par un abaissement de la tension nicessaire B l'observation du claquage par avalanche ainsi que par une localisation du mtcanisme de multiplication au sein des zones actives qui conduit B une augmentation de rendements de conversion (17% a 100 GHz). La diminution des tensions de fontionnement est confirmee expkrimentalement par des mesures priliminaires de mux d'ionisation effectuies sur hit6roipitaxies GaAslALGaAs de type puits quantiques Clabories par ipitaxie par jet moleculaire.
Abstract Miscroscopic simulations of impact ionization in GaAsIAIGaAs heterostructures have been performed to predict the potential performances of multilayered avalanche and transit time oscillators for a 100 GHz operation. It is .shown the capability of controling and assisting the carrier emission by impact ionization. This yields a lowering of the breakdown voltage and a confinement of the multiplication inside the active zone which leads to an increase of the conversion efficency (17% at 100 GHz) of avalanche millimeter sources. The reduction in the operating voltage is confirmed experimentally by preliminary ionization rate measurements carried out on GaAslAlGaAs multi quantum wells grown by molecular beam epitaxy .
I - Introduction
A fundamental difficulty to generate power with avalanche and transit time devices (IMPATT diodes) in the millimeter wave frequency range is a marked reduction of the non linearity of the avalanche process as the frequency goes up. Basically, this reduction is due to the following fact : since carriers must be accelerated in the electric field until they reach the ionization energy and therefore impact ionize, it may be very difficult to achieve this over travelled distances shorter than a micron and rapid time variations. Recent progress in epitaxy technique raised the possibility of fabricating multilayered heterojunctions. In such structures, the band gap may be varied and as demonstrated by several authors "band gap engineering" can be used to assist the carrier multiplication by impact ionization [l] [2]. At the moment, this concept has been applied almost solely to avalanche photodiodes in order to reduce avalanche noise. Some analysis of superlattice mixed avalanche and tunnel transit time diodes [3] have been proposed but otherwise relatively little attention has been paid to the application of band gap engineering to avalanche transit time devices. In this paper, we theoretically and experimentally investigated this research area by concentrating on GaAslGaAlAs structures. Theoretically, our aim was to depict impact ionization in GaAsIGaAlAs heteroepitaxies. Time domain simalations were performed to predict the potential performances of multilayered avalanche and transit time oscillators for a 100 GHz operation. This theoritical work will be presented in section 11. Section I11 will be devoted to preliminary measurements of impact ionization rates in multiquantum well structures grown by MBE. Concluding remarks will be given in section IV.
2 - Simulation
The Monte Carlo method has been used extensively in the study of electron transport in semiconductors.
Only a brief discussion on how the ionization rates are calculated will be given here. The method consists of simulating the motion of carrier in wave vector space. The motion consists of free flights under the action
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19875103
C5-488 JOURNAL DE PHYSIQUE
of an applied electric field separated by scanering events. Therefore, the simulation procedure requires the knowledge of the band structure and of the differents scattering mechanisms that a carrier undergoes during its motion. The rather complex conduction band structure was calculated by the empirical pseudo potential method. Figure 1 illustrate the isoenergy lines in the rXK cross section obtained by this method for an aluminium concentration of 0.45. The high energy states are given in shaded zones. One can note that theses zones are located outside the main symetry lines only described in simple dispersion relations. This shows the importance to depict high energy sites. The next step in the simulation is the derivation of the scattering rate. The scattering mechanisms described are acoustic phonon, polar optic01 phonon, intervalley scattering and alloy scattering. Most of the parameters necessary to calculate the scattering rate were taken from reference [ 6 ] . Impact ionization is treated as an additional interaction. Physical parameters such as the mean ionization length (Ai) are calculated by taking average over successive free flights and scattering mechanisms. The ionization coefficients a is the reciprocal of (Ai) over a sufficient number of ionization.
Figure 2 shows the electric field dependence of the calculated electron ionization rate for two AI concentrations often encountered in pratical applications x=0.25 and x=0.45. For comparison GaAs rates are also presented.. For electric field values below 500 kVlcm, it is found that no significant ionization is observed in GaAlAs. Above this value, the ionization rate increases. In the hight electric field limit, the calculated values become comparable to those in GaAs. Analysis of energy distribution reveals that this result is explained by the fact that the electric field is strong enough to defeat the scattering interactions and thus elevate the electron energy to a high energy state. From the knowledge of ionization rates upon electric field and AI concentration we depict now device characteristic. The numerical procedure that we used to perform the study of multilayer avalanche and transit time diodes in the time domain was derived from a simplified particle simulation described previously [4][5]. In this model each carrier has a probability Pi of undergoing an ionizing collision which is a function of its energy and of the material in which the carrier is located. This probability is directly related to the carrier ionization rate calculated in the first section. The energy dependence is obtained by fitting the steady state results of energy and ionization versus electric field
Figure 1 Isoenergy lines in the first conduction Figure 2 electric field dependence of the calculated band in rXK cross section (GaAIAs, X = 0.45) electron ionization rates
Two kinds of structures, have been simulated. The first structure is a double drift heterostructure. Fig 3 shows typical information on the carrier distribution versus time and distance which can be obtained by means of the numerical procedure. In the example the AI content is 0.4. The spacing between the heterointerface is 1000 A. A rather sharp electron bunch is here generated by impact ionization in the middle zone of the active region giving rise by transit effect to the appearance of oscillations near 100 GHz. This indicates a localization of the multiplication region by the two GaAlAs zones adjacent to the central GaAs region. RF performances calculated on non optimized structures predict encouraging performances, typically 17 % of intrinsic conversion efficiency at 100 GHz for a bias current density of 29 kA/cm and a bias 2 voltage of 20.5 V. The second structure is an avalanche region integrating multiquantum wells. In this case,
of the direction of its motion. The fact that the ionization probabilities via the ionization rates are non linear functions of the carrier energy implies that higher values of ionization rates are obtained in hetero multilayers with respect to conventionnal bulk material. Extensive theoritical analysis of these phenomena were recently published by BRENNAN [2]for low values of applied electric field by assuming a GaAs band structure shifted in energy in GaAlAs material. Figure 4 shows the results obtained for the dependence of the electron impact ionization rate as a funtion of well width at a high value of electric field (E=500kVlcm )obtained with the numerical procedure outlined before. As the period of the multilayers decreases the ionization rate inscreases. This can be explained by the fact that the electrons see many more potential steps.
For a well width of 100 A the ionization rate in multiheterostructures is 2,5 times higher than the GaAs rate. Physically, this corresponds to an apparent lowering of the ionization threshold which yields a reduction of the applied voltage necessary to observe the onset of avalanche breakdown. From a device point of view, let us recall that one of the limitation of conventionnai IMPATT is due to the dissipation in the avalanche zone, of the power part which is not converted in microwave power. On this criterion, the lower avalanche region voltage leads to an increase of the conversion efficiency.
Figure 3 Spatial distribution of electron densities in ~i~~~ 4 ~~~~~d~~~~ of electron impact a quantum well avalanche and transit time diode at ionization rate as a function of well width+
various times of a 100 GHz operation.
l:
;:
0
'; .- C N 7
6 S -
.-
z B 5 -
3 - Experiment
Experimentally, GaAs - AI,Ga,-,As multiquantum wells with large well and barrier widths (100 di - 500
A) were grown in a MBE 2300 Riber system. < 1,0,0> ,n type, silicon doped GaAs wafers are used as +
substrates. The entire structures were grown with a substrate tem erature T=~OO'C. The AI concentration
15 P
was 0.4. The doping level is low estimated at about 2x10 cm- to ensure a spatially uniform field. To determine the quality of the multiquantum well materials, photoluminescence measurements were made at 2 ' ~ . The excitation is performed using the 5145 A line of an Argon laser. In figure 5 is presented the typical spectrum obtained on the 500 A and 100 A samples. A peak appears at 1,519 eV fo; the 500 A sample and at 1,594 for the 100 A sample. The confinement effect in energy in the 100 A sample is therefore displayed. The linewidth of the photoluminescence varies in the range 4 to 6 mev and reflects the
\
m
E = SOOkV/cm:
-
L
quality of the layers.
200 400 600. 800
well width. A
From the layers, circular etched mesa photodiodes were formed using standart lithographic technics.
Schottky Pt contact was formed by sputtering on the top of expitaxies followed by a TiPrAu metallization for the external conctact. An ohmic GeAuNi was formed on the back side. The mesa diameter was 250 pm.
Fig 6 shows a lateral view of mesa etch obtained by SEM micrograph. The alternating layers are revealed here by the difference of etching between GaAs and GaAlAs layers. The cross section device structure is shown in the inset of this figure. The typical reverse I(V) characteristic is shown in figure 7. The breakdown voltage Vb (definied at a current of 1 wA) is -29V. The dark current is < I n.4 at 0,5 Vb. The determination of impact ionization rates were then performed using photomultiplication and noise characteristic following the measurement method given in ref [7]. In most of the multiquantum well structures a lowering of the avalanche voltage correlated with a reduction of the effective ionization threshold as described in the first section is found. In table 1, we give the estimation of electron and hole ionization rates obtained for the structure described previously at the onset of breakdown. For comparison the GaAs rates values taken from ref [S] for the same equivalent electric field value are also given. Concerning the electron ionization rate, a significant increase in the electron ionization rate in agreement with the theoritical
C5-490 JOURNAL DE PHYSIQUE
analysis and previously reported values is obtained. The hole ionization rate are almost unchanged within limits of experimental error.
Conclusion
In this paper, two examples of Band gap engineering applied to avalanche transit time devices (IMPATT) shows the capability to increase the conversion efficiency of millimeter sources by controling and assisting the carrier emission by impact ionization. The cases which were discussed correspond to heterostructures with large widths of barrier. The yields a coupling between wells negligeable. As the dimensions of heterobarriers are decreased, the probability of tunneling through the barrier increases. This gives rise to the appearance of new operating conditions such as mixed avalanche and tunnel emission and energy filtering.
In this case, it is expected that the frequency performances should be improved opening a new way in the search of modulated semiconductor high frequencies sources.
o MOW 500-500A
b M O W 100- 100A
1 1.52 1.53 .. energy 9 . b ~ ( e V I 1.55 - - J
Figure 5 Photoluminescence spectra
Figure 6 SEM lateral view of mesa etch.
ionization rates 103cm-l
Gcl As 5.8 3.3
Table 1 Ionization rates at breakdown voltage Figure 7 Reverse I(V) characteristic. for 500 A GaAslAlGaAs MQW structure.
Comparison with GaAs.
References
[l] F. Capasso Semi conductors and semi metals, Vol 22, Part d.
[2] K. Breman Tran. an electron devices, Vol. ED-33 10,1986 131 A. Christou and K. Varmaziz Appl. Phys. lett 48(21) 1986
[4] D. Lippens, J.L. Nieruchalsky, E. Constant IEEE Trans. an electron devices, Vol ED-32,11 1985.
[S] D . Lippens, J.L. Nieruchalsky Appl. Phys. Lett 48(21) 1986.
[6] S . Adachi, J. .Appl. Phys. 58(3) 1985
[7j H. Ando, M.Kanbe Solid State Electron. Vol 24 p 629 1987
[g] G.E. Bulman, V.M. Robbins, G .E. Stillman IEEE Trans. an Electron Dev Vol ED-32,11,1985
