Demonstration of the N2 carrier process for LP-MOVPE of IIIV's

,. . . . . . . .

ELSEVIER

CRYSTAL GROWTH

Journal of Cl-yst~.ll Growth 170 (1997) 103- I ()g

Demonstration of the N 2 carrier process for LP-MOVPE of

III/V's M. H o l l f e l d e r ~, S. H o n ~', B. Setzer ~, K. S c h i m p f a, M. H o r s t m a n n ~ Th. Sch~ipers ~' D. Schmitz t', H. Hardtdegen ~, H. Ltith ~' ~' Instimt.ffir .S'ctHcht- mM hmemectmik, Fm'.~clmn~.,szcmrum Jii/ich, D-52425 Jiilich, Germcmy ~' A IXTRON Semiconductor Tect mdo vie~ GmbH. Kc.'/~erlstrcl.~'se 15 ] 7, D-52072 Aachen. Germany

Abstract

The suitability of an N~ carrier in LP-MOVPE of G a I n A s / l n P device structures and for the growth of (AI)GalnP is investigated for the first time. Al-free G a I n A s / l n P HEMTs and MSM photodetectors exhibit cutoff frequencies of .f) - 135 GHz and ./i..... = 200 GHz and a bandwidth of 16 GHz and responsivity of 0.27 A / W , respectively. A1GaInP and GaInP layers deposiled using the optimized growth conditions showed excellent structural, optical and homogeneity properties. For example X-ray diffractograms with FWHMs as low as 15-16 arcsec for 1 /_tin thick layers and 300 K photoluminescence mappings over full 2 inch wafers with standard deviations of + 0.23 and + 0.26 nm were obtained for both materials.

1. I n t r o d u c t i o n

Large scale production of I I I / V device structures is done preferentially by MOVPE. Industrially, reactors with capacities of up to 95 two inch wafer are employed for the deposition of such structures. Particularly for LED application enormous amounts of carrier gas, usually hydrogen, and source chemicals, especially AsHs and PH 3 are consumed during deposition. Every contribution, which develops the MOVPE process in terms of an increase in process safety, malerial quality a n d / o r a reduction of process costs, is of great importance for the whole manufacturing process. These advantages have been demonstrated in the past for the employment of an N~ carrier with respect to AIGaAs/GaAs and

GaInAs/InP growth [1 4]. In this paper we will for the first time investigate the suitability of the N, process for the growth of GaInP and A1GalnP with respect to HBLED application. Up to now it has not been shown, that device properties of structures deposited with the N 2 process are competitive to those deposited with the H~ process. In the second part of the paper we will report on the device properties of GaInAs/InP HEMTs and MSM photodetectors grown with an N, carrier. The influence of the carrier gas in MOVPE is basically due to changes in the hydrodynamics of the growth process. Assuming the same growth conditions, the exchange of the usually employed H, carrier for N 2 results in a reduced boundary layer height and an increased entrance length and lower

0022 (.1248/97/$17.00 Copyright ',~'~1997 Elsevier Science B.V. All rights reserved Pll S 0 ( } 2 2 - 0 2 4 8 ( 9 6 ) 0 0 5 4 5 - 3

104

M. Hol!/k, lder et al./,lotlr, lal ~/' Crv.~tal Growth 170 (1997) 103 1()~%'

diffusion coefficients for the precursor molecules in the gas phase. As a consequence, the homogeneity of the layers is significantly improved.

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% 2. Experimental procedure All layers were grown in a conventional horizontal MOVPE reactor (AIXTRON, AIX 200) using gas toil rotation and the standard precursors, TMAI, TMGa, TMIn, arsine and phosphine. Additionally, the N 2 carrier gas was purified by means of a getter column (SAES Getters). Deposition was carried out at a gas velocity of 0.9 m,/s at a reactor pressure of 20 mbar and 50 mbar for G a l n A s / I n P and (Al)GalnP, respectively. HEMTs and MSM-PDs were grown on semi-insulating Fe-doped InP substrates, exactly oriented along the (100), at a temperature of 670°C with a V / I l l ratio of 250 for InP and 80 for GalnAs. DMZn was used for p-type and SieH 6 for n-type doping. (AI)GalnP was deposited on GaAs (100) substrates, misoriented by 2 ° towards (110) and by 6 ° towards (111)A with a 300 nm thick GaAs buffer layer. Growth temperature and V / I l l ratio were varied from 600°C to 800°C and from 50 to 350, respectively.

3. The growth of (AI)GalnP Deposition of Ga0.52In0.4s P and (A10 ~Ga0.v)0.52In0.4sP was optimized by variation of growth temperature, V / I l l ratio and reactor pressure. 50 mbar is the most suitable reactor pressure for growth in a nitrogen atmosphere. Good quality material can be grown at 740-780°C and at a V / I l l ratio of above 100 and above 50 for Ga052In0.4~P and (A10.3Ga0.7)052In0.4,P, respectively. For 1 /xm thick layers a FWHM of the (004) X-ray reflection for both materials of only 15-16 arcsec was obtained, which are the best values ever reported to our knowledge. The excellent structural quality and homogeneity of composition in growth direction is depicted in Fig. 1. Optical properties were investigated by 2 K photoluminescence. GalnP layers grown on (00l)-oriented GaAs exhibit ordered domains. The degree of ordering determines the band gap of GalnP and

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Fig. 1. X-ray diffractograms of I [Lln thick Gal~,lnoa~P and (Alll.~Gal,.7)llszln0asP layers deposited with an N, carrier using the optimized growth conditions.

depends substantially on the growth temperature due to the formation of the CuPta-type (GaP)~(InP), monolayer superlattice structure on the {111} lattice planes [ 11 ]. We found the same dependence of ordering on growth temperature as for layers grown in an H~ ambient [12]. Further the misorientation of the substrates strongly influences the degree of ordering as it is pointed out in Fig. 2. misorientation towards the opposite orientation of the monolayer superlattice structure reduces the ordering effects and therefore

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Fig. 2. 2 K photoluminescence of Ga052 Ino4 s P and (Alo3Gaii.7)o.52|no.4~P layers deposited with an N~ carrier using the optimized growth conditions and substrates with different degree of misorientation.

M. ttoll/~'lder et al./,Iourplal ~/"C/Tstal Grm*'th 170 (1997) 103-108

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Wafer position [mm] Fig. 3. 300 K photolummescence mapping of (a) GalnP and (b) AIGalnP over a full 2 inch wafer.

M. Hol(/k, ldcr ctal. ~.Iotarod 01 Cry.sfal Growth 170 (1997) 103-l(),S'

106

increases the band gap. The FWHM of the photoluminescence peak is a figure of merit for the optical quality of the samples. The FWHMs determined from the photoluminescence spectra in Fig. 2 are state of the art and demonstrate the excellent optical quality of the samples deposited using the optimized growth conditions. The homogeneity of the layers a over full 2 inch wafer sparing out only the outer 3 mm rim was studied by photoluminescence at 300 K. The results are presented in Fig. 3a and 3b for Ga0.52In04sP and (A10 ~Ga07)05_~ I n0.4s P, respectively. We obtained a min to max variation of the peak wavelength of _+ 1.2 nm for both materials, a uniformity which has up to now not yet been obtained in single wafer horizontal reactors with an H 2 carrier. With respect to industrial applications, i.e. HBLEDs, a standard deviation of the PL peak wavelength should be less _+1 nm over the whole wafer. We determined a standard deviation of _+0.26 nm and +0.23 nm for Ga05elnoasP and (Al0~Ga07)0.5~lnll.4sP, respectively, i.e. nearly the complete water area can be employed for LED production. These contributions demonstrate the suitability of the N, carrier for the fabrication of G a l n P / A I G a l n P structures. 4. G a l n A s / I n P

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frequency [GHz] Fig. 5. Schematic structure (a). current gain h, I and unilateral power (GU) (b) extractcd flom S-parameter measurements oI an optimized 0.2 t,tm T-gate HEMT.

device structures

The smoothness of the hcterointerface as well as the purity of the bulk layers plays an important role T=IK

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for the transport properties of devices such as HEMT's. The optical and electrical quality of bulk GalnAs and InP layers, grown with an N, carrier, has been investigated in the past [1]. Here, the smoothness of the heterointerface was investigated. To this end modulation doped 2DEG structures containing a 10 nm strained GaInAs channel (_vh~ = 77%) were deposited first. Fig. 4 shows Shubnikov-de Haas oscillations and Quantum Hall effect measuremeats along with the schematics of the 2DEG structure under investigation. The excellent transport properties are documented by an extremely high electron mobility of 460000 c m - ' / V . s at 1.0 K, occupation of only one subband and no conducting bypass channel. Similar structures grown with an H carrier exhibit nearly the same electron mobilities [5,6], pointing out that the N~ carrier is equally suitable for the growth of mesoscopic systems and obtaining smooth heterointerfaces.

M. Hollfi'ldcr et al. /,Iotovtal (?f CO'sial Growth 170 (1997) 103- 108

4.1. HEMTs

4.2. MSM-PDs

For our Al-free G a l n A s / I n P HEMT we used a highly strained GalnAs channel (.v~,, = 77%) to improve the carrier confinement and transport properties. The gate to channel separation was kept as small as possible to avoid short channel effects at small gate lengths. Reduction of the gate to channel separation is limited by the thickness of the p-doping layer used for Schottky barrier enhancement [7] and by Zn diffusion, since the diffusion of Zn towards the n - supply layer and the channel needs to be avoided. A schematic structure of the optimized G a l n A s / l n P HEMTs is depicted in Fig. 5a. Details of the device fabrication are reporled in Ref. [8]. Typical DC data of a 0.2 × 100 # m 2 device with a source drain spacing of 2.5 p,m are a maximum drain current of 600 m A / m m and a maximum transconduclance of 500 m S / m m at 2 V drain bias. Current gain /12i and unilateral power (GU) extracted from S-parameler measurements of an 0.2 /,m T-gate HEMT are displayed in Fig. 5b. Cutoff frequencies of .fl = 135 GHz and ./i..... = 200 GHz were determined by extrapolation to 0 dB which are the best values ever reported for aluminum-free G a l n A s / I n P HEMTs [9]. 0.0

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5. Conclusions -3dB=16G 20 nm GalnAs n=2xl0'°cm "° 15 nm InP p=2x1018cm "3 15 nm InP separation 150 nm GalnAs absorption 10 nm GalnAs xln=77% chann~

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f r e q u e n c y [GHz] Fig. 6. Schematic structure and frequency response of an optimizcd M S M PD structure with 0.5 /xm finger-spacing and width, without A R - c o a t i n g at a wavelength of 1.3 p,m and a PD bias of 5

V.

G a l n A s / I n P MSM photodetectors with current transport along the 2DEG were fabricated using a similar layer structure and processing procedure as employed for tile HEMTs. This concept offers new feasibilities to realize monolithically integrated photoreceivers. Fig. 6 presents the schematic structure and the frequency response of the optimized MSMPD. The responsivity depends oll the thickness of the GalnAs absorption layer and aspect ratio of fingerspacing to finger-width and the speed is limited by the device capacitance and the transit time of the photogenerated carriers through the layers. Further details on the device concept and preparation can be found in Ref. [10]. At a wavelength of 1.3 /,m without AR-coating, a PD bias of 5 V and with 0.5 p,m finger-spacing and width the MSM-PDs exhibit a 3 dB bandwidth of 16 GHz and a responsivity up to 0.27 A / W . The bandwidth is limited by the carrier pile up at the G a l n A s / I n P interface between absorption layer and separation layer. The measured bandwidth is among the best ever reported for GalnAs MSM-PDs and points out the suitability of an N~ carrier for the growth of high frequency and optoelectronic devices.

,

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•

107

We investigated the device properties of G a l n A s / I n P HEMT and MSM-PD structures and the optical and structural characteristics of (A1)GalnP bulk material deposited by LP-MOVPE using an N~ carrier gas. The excellent device performances of the HEMT and MSM-PDs demonstrates the suitability of the N 2 process for the growth of high frequency and optoelectronic device structures. We obtained (A1)GaInP bulk materials with excellent structural and optical properties. The state of the art uniformity of the PL peak wavelength meets by far the requirements for industrial applications and again verifies the advantages of the N 2 carrier process with respect to improved material characteristics at increased process safety and reduced process costs.

1(18

M. Holllblder et al. /,lournal (~/'Co:vtal Growth 170 (I 997) 103 I 0~

References [1] M. Hollfelder, S. Hon, H. Hardtdegen. M.v.d. Ahe. R. Carius and H. Li.ilh, in: Proc. 6th Eur. Workshop on MOVPE and Relaled Growth Techniques, Gent. Bclgium, 1995, p. DI3. [2] M. Hollfelder, H. Hardtdegen, R. Meyer, R. Carius and H. Li_ith, J. Electron. Mater. 13 (1994) 603. [3] H. Hardidegen and P. Giannoules, 111 Vs Review 8, No. 3 (1995) 34. [4] D. Schmitz et al., Poster Presentation, ICMOVPE 6, Cambridge MA, USA, 1992. [5] H. Hardtdegen, R. Meyer, M. Hollfelder, Th. Sch:@ers, J. Appenzeller, H. Loken-Larsen, Th. Klocke, Chr. Dieker, K. Schmidt, B. Lengeler and H. Liith, J. Appl. Phys. 73 (1993) 4489. [6] P. Ramvall, N. Carlsson, P. Omling, L. Samuelson, W.

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[10]

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Seifen. M. Stolze and Q. Wang, AppI. Phys. Lctt. 68 (1996) 1111. P. Kordos, M. Marso. R. Meyer and H. Lfifll, J. Appl. Phys. 72 (1992) 2347. M. Marso, K. Schimpf, A. Fox, A.v.d. Hardt, H. Hardldegen, M. Hollfelder and P.Kordos, Electron. Lelt, 31 (1995) 589. A. Mesquida Kiisters. T. Funke. V. Sommer, R. Wi]ller. S. Brittner, A. Koh[ and K.Heime, Electron. Lett. 29 (1993) 841. M. Horstmann, M. Marso, A. Fox. F. Riiders, M. Holffekler, H. Hardtdegen, P. Kordos and H. Liith, Appl. Phys. Lett. 67 (1995) 106. J.B. Bernard, S. Fro~,en and A. Zunger, Phys. Re,,'. B 45 (1992) 11173. M. Kondow, H. Kakibayashi and S. Mmagawa. Appl. Phys. Lett. 53 (1988) 2053.