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HIGH PERFORMANCE SCHOTTKY DIODE AND FET ON InP
S. Loualiche, A. Ginoudi, H. l’Haridon, M. Salvi, A. Le Corre, D. Lecrosnier, P. Favennec
To cite this version:
S. Loualiche, A. Ginoudi, H. l’Haridon, M. Salvi, A. Le Corre, et al.. HIGH PERFORMANCE
SCHOTTKY DIODE AND FET ON InP. Journal de Physique Colloques, 1988, 49 (C4), pp.C4-217-
C4-221. �10.1051/jphyscol:1988445�. �jpa-00227943�
JOURNAL DE PHYSIQUE
Colloque C4, supplBment au n09, Tome 49, septembre 1988
HIGH PERFORMANCE SCHOTTKY DIODE AND FET ON InP
S. LOUALICHE, A. GINOUDI, H. L'HARIDON, M. SALVI, A. LE CORRE, D. LECROSNIER and P.N. FAVENNEC
Centre National d'Etudes des T6l~connnunications LAB/OCM/TOH, B P 40, F-22301 Lannion Cedex, France
RBsumB
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Une diode Schottky de trgs haute qualit6 a Bt6 rBalis6e sur InP de type n en utilisant un traitement de surface original.Cette diode atteint des tensions de claquage de 60 V et le courant de fuite reste inf6rieur h 1 pA h 30 V. Le meilleur dispositif a un courant de fuite de 0.2 nA d
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1 V et une hauteur de barrigre de 0.7 eV. Cette Schottky a Bt6 utilisge pour la fabrication de transistors h effet de champ (FET) par CBE et implantation ionique. Une transconductance de 140 mS/mm a BtB mesurge sur un transistor de 3 pm de grille.Abstract
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A high performance Schottky diode has been realized on InP by a special surface treatment. The diode reaches a breakdown voltage of 60 V and the reverse current remains at 0.6 PA for 30 V reverse voltage. The best device shows a reverse current of 0.2 nA at 1 V voltage with an ideality factor of 1.54. The Schottky has been used as a gate in the fabrication of FETs on InP by ion implantation and CBE. A transconductance of 140 mS/mn has been obtained on a 3 pm gate length FET in the frist experimental trial without any optimisaton.The Schottky diode is widely used in high speed devices like FETs and optoelectronic devices like MSM (metal semiconductor metal). The InP and related ternary and quaternary materials are the most important semiconductors for the optoelectronic technology. These materials are used in the fabrication of emitter and receiver devices in the 1.3 pm and 1.5 p m wavelengths for lightwave communications in opticalf fibers.
The InP and related compounds have also excellent transport properties (high mobilities and high saturation velocities) for high speed applications. But due to the absence of good quality Schottky diodes,
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1988445
C4-2 18 JOURNAL DE PHYSIQUE
this material is not extensively used in high speed devices (FET, HEMT) like GaAs.
In the present work we have fabricated a good quality metal insulator semiconductor (MIS) tunnel Schottky device on InP and GaInAs. The fabrication has been done by a low temperature process (below 200%) which leads to excellent devices.
The samples used to study Schottky diodes are undoped n type (5 x 1015
~ m - ~ ) InP substrate or layers grown by gaz source molecular beam epitaxial layers (also called CBE : chemical beam epitaxy). The CBE layers are n type InP (undoped
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1016 cm-3 or n type GaInAs ( - 5 x 1015~ r n - ~ ) of about 1 to 3 m thickness grown on semi-insulating fnP : Fe substrates. The ohmic contact (Au-Ge annealed 360°C
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10s) is done first to avoid the annealing step to the Schottky contact. After the oxide fabrication, the Schottky metal (Au) is deposited through a metalic mask. The Schottky dots have two dimensions : 0.2 mm and 0.4 mm. The carrier concentrations are measured by C(V) technique using the Schottky device.The current versus voltage (I(V)) electrical characteristic of a typical diode measured at: room temperature is given in Figure 1. The breakdown voltage of this diode is about 10 V, the ideality factor is 1.54 and the apparent barrier height is 0.7 eV. The reverse leakage current at 1 V is below 0.2 nA. To the best of our knowledge these parameters seems to indicate that this Schottky diode is the highest quality device ever fabricated on n type InP and campares favorably to the Schottky diodes fabricated on GaAs. The study of the saturation current with temperature from 100 K to 300 K ( RQf
.
1 ) leads to evaluate the oxide thickness of the MIS tunnel diode of the figure 1 to 35 A. By increasing the 0temperature of the diode fabrication by 50'~ above that of figure 1, the breakdown voltage is increased from 10 V (Figure 1) to 60 V (Figure 2).
,O.S ] 1 (A)
hP:n 0,- 0.7 eV
n = 1.51 VBR =lOV
10'" 0 0.5
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1.0Figure 1 : Typical I(V) characteristic of 0.2 mm diameter diode on n : InP
Figure 2 : Reverse I(V) characteristic of 0.2 mm diameter diode on n tvDe 4 r
InP. The leakage current 1s about 0.6 PA a t 30 V.
The barrier height of this device is 0.65 eV and the ideality factor is 1.45
.
The reverse leakage current of this diode is still below the microampere range at 30 V reverse voltage (Figure 2). The oxide thick- ness of this diode is estimated to 50 A by I versus T measurement. An 0ellipsometry evaluation of the oxide thickness leads to 25 A for the 0
device of Figure 1 and 35 A for that of figure 2. An other method has 0
been used to evaluate the oxide thickness fabricated on InP by the present oxidation method. A part of on an n type InP sample has been protected by 100 A Pt layer on its top, the other part has been oxidised 0
and etched ten times to measure the thickness of the InP material used by the oxidation. These measure gives 200 A for the InP removed by 10 0
successive oxidation and etch runs. This experiment leads to an oxide thickness of 40 A if we suppose that toxide 0 = 2 t ~ ~ p . These three
different methods give an order of magnitude of the optimum oxide thickness for the Schottky contact of 40 A. 0
The oxide fabricated to enhance the surface barrier of the Schottky device seems to be stable. The Schottky device parameters does not change when the diode is stored at room temperature during a few months.
The device has been polarised at 3 V reverse voltage during 48 hours and we do not observe any particular variation of the device characteristics. The annealing at 200°C for 10 minutes of the finished diode decreases the leakage current by a factor 2. The annealing of the oxide prior to the Schottky metal deposition at 2 0 0 ~ ~ for 30 minutes im- proves the Schottky quality (better leakage current). At the temperature of 250'~ and above the oxyde quality is degraded.
The gold (Au) has been chosen for the Schottky contact. Other metals (Al, Ag, Cr) are also tested, but the devices fabricated by te gold contacts give better performances.
A Deep Level Transient Spectrocospy (DLTS) measurement has been performed on the Schottky fabricated on undoped InP. The temperature range of the measurement lies between 90 K and 300 K and no trap has been observed in this temperature range. If we suppose a trap emission cross section of 10-l5 cm2, there is no trap, having its energy lying between 0.12 eV and 0.6 eV below the conduction band, introduced by the Schottky preparation technique. The experimental sensitivity, indicates that the trap concentration, if they exist, is below 1010 cm-2.
The present oxidation method studied on n type InP has been tested on GaInAs layers grown by CBE on InP substrate. The GaInAs material in n type (
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5 1015 ~ m) "undoped having about ~ 1 m thickness. The Schottky fabricated on this material has higher leakage current.but the breakdown voltage is as high as 300 V (Figure 3). This is the highest breakdown voltage ever obtained on a Schottky device in GaInAs semiconductor.JOURNAL DE PHYSIQUE
F i g u r e 3 : L i n e a r I(V) c h a r a c - t e r i s t ~ c of a S c h o t t k y diode o n n t y p e GaInAs ( 1 prn thickness : 5 1015 cm-3) g r o w n o n a s e m i Insulating InP s u b s t r a t e . N o t e t h e breakdown v o l t a g e ( > 300 V).
The Schottky device fabricated on n type InP has good long term stability, does not introduce deep traps and the device parameters are suitable for electrical measurements ( I ( V ) , C(V), DLTS). This Schottky device has been also found suitable for the gate contact in field effect transistors (FET) fabricated on n type InP.
Two methods have been tried to dope the active layer of the transistor : Ion implantation and CBE. The silicon ion implantation on <loo> semi insulating substrate at 2 doses and 2 energies (1012
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40 keV and 2 x 1012-
100keV). To improve the ohmic contact a shallow n+
ion implantation (1013
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20 keV) has been done below the source and drain contact. The ion implantation annealing is performed at 7500C during 30 seconds.The ohmic contact annealing occurs at 340'~ for 15 seconds. Before the fabrication of the Schottky gate, we have done a 1200 A etch below the 0
gate by a chemicekl solution (Br2 : HB,: H20 : 0.1 : 10 : 100) for the control of the saturation current. The gate Schottky contact has higher leakage current compared to the previous Schottky diode. For a 3 p m x 300 y m gate contact the leakage current is about 1 pA at
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1 V and 4 p Aat
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2V. The best transconductance on a 3 y m gate implanted FET is 140 mS/mm and compares favorably to the best 1 p m gate MIS devices on InP (180 mS/mm : Ref. 2-
200 mS/mm Ref. 3). But the device with the best transconductance does not reach the pinch off. The device reaching the pinch off has a transconductance of 110 mS/mm for a 3 p m gate. Other devices presents lower transconductances (Figure 4), but these devices can support Vds voltage above 11 V, and have excellent and constant output transconductance from Vgs = 0 to the pinch off. This property make them suitable for high power amplification with a good linearity.The active layer grown by CBE consists of an undoped buffer layer (0.5 ym ; 1016 c ~ n - ~ ) and a doped active layer (0.2 pm : 2 x 1017 c V 3 ) has been grown to improve the ohmic contact. The contact resistance is found to be better in CBE FET (3.5 5 1) than in the implanted one (30 52 ). It is necessary to etch under the gate to control the drain saturation current and to reach the pinch off of the transistor because of the high residual doping of the buffer layer. We have found an other methods to
0
avoid the etch by growing a p type layer (500 A ; 3 x 1017 cm-3) between the buffer layer and the active layer. This method leads to transistors which reach the pinch without etching below the gate for a moderate gate voltage of 3 to 4 V. The gate leakage current is about 4 p A at
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3v.
The transconductance obtained in the case of CBE transistors is less than that obtained on implanted FET (50 mS/mm ; Figure 5). This is probably due to the poor quality of our buffer layer.
F i g u r e 4 : Ids
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Vds network of ion lrnplanted F i g u r e 5 : Ids-
Vds n e t w o r k of CBE FET FET o n n t y p e InP g a t e d i r n e n s ~ o n g a t e dimension : 3 prn x 300 pm3 prn x 300 prn
In conclusion, a high quality MIS tunnel Schottky diode has been fabricated on n type InP. This device presents excellent long time stability, high breakdown voltage (60 V), and ultra low leakage current (0.2 n A ) . The fabrication process does not introduce deep traps. This Schottky diode is adequate when it is used as a gate in the FET. We have shown that the fabrication process of the diode is compatible with ion implanted technique or epitaxial technique for FET fabrication.
REFERENCES
[I] S.M. Sze, Physics of semiconductor devices, (Wiley, New York, 1981) p. 547
[2] A. Antreasyan, P.A. Garbinski, V.D. Mattera, H. Tenkin, J.H. Abeles, Appl. Phys. Lett.
2 ,
1097, 1987[3] T. Itoh, K. Ohata, IEEE Transaction on Elect. Dev. ED 30, p. 811, 1983
