Reduction of autodoped gallium concentration in HgCdTe layers on GaAs grown by metalorganic vapor phase epitaxy

......... CRYSTAL GROWTH

ELSEVIER

Journal of Crystal Growth 146 (1995) 619-623

Reduction of autodoped gallium concentration in HgCdTe layers on GaAs grown by metalorganic vapor phase epitaxy H. Nishino *, S. Murakami, H. Ebe, Y. Nishijima Fujitsu Laboratories Ltd., 10-1 Morinosato-Wakamiya, Atsugi 243-01, Japan

Abstract

We studied the mechanism of Ga autodoping in HgCdTe layers grown on GaAs by metalorganic vapor phase epitaxy (MOVPE). We determined growth conditions suitable for reducing the carrier concentration in unintentionally doped HgCdTe layers. We detected Ga atoms in HgCdTe layers rather than in CdTe buffer layers. Ga atoms did not diffuse into HgCdTe layers from GaAs substrates through CdTe buffer layers, but were incorporated from the vapor phase during HgCdTe growth. We assumed that the GaAs back surface reacted with Hg and metalorganic precursors during HgCdTe growth and this reaction produced volatile organic-gallium which acted as the Ga autodoping source. To confirm this, we coated the GaAs back surface with Si3N 4 and obtained HgCdTe layers with carrier concentrations below 10 ]5 cm -3. We also clarified the role of Hg in the incorporation mechanism. Because the concentration of both carriers and Ga atoms increased with Hg partial pressure, Hg behaved as a catalyst for the organic-gallium production. Using layers with low Ga concentrations, we fabricated photodiodes whose performance was as good as those fabricated using liquid phase epitaxial (LPE) grown HgCdTe/CdZnTe layers, because the tunneling current due to the residual donor concentration was suppressed.

I. Introduction

Heteroepitaxial growth of CdTe or H g C d T e on G a A s has been studied for the use in fabricating large-area infrared focal p l a n e arrays (IRFPA). Metalorganic vapor phase epitaxy ( M O V P E ) is one of the most promising techniques for growing HgCdTe, because of its high throughput and suitability for large-area substrates. To realize I R F P A using G a A s substrates, there are some obstacles that must be overcome.

* Corresponding author.

High-quality long-wavelength infrared (LWIR) photodiodes require H g C d T e layers with a low dislocation densities (less than 3 x 10 6 cm -2) [1]. However, the dislocation density in H g C d T e layers grown on G a A s substrates is high because of the large lattice mismatch (14.6%) between the G a A s and CdTe buffer layers. We have studied the dislocation reduction mechanism in H g C d T e on GaAs grown by M O V P E [2] and obtained H g C d T e layers with etch-pit densities (EPDs) b e t w e e n 10 6 and 3 X 10 6 cm -2. We believe that this is low enough for photodiodes. Another problem is the contamination of MOVPE-grown H g C d T e layers. It is difficult to obtain H g C d T e layers with carrier concentrations

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H. Nishino et al. / Journal of Crystal Growth 146 (1995) 619-623

below 10 t5 c m - 3 o n GaAs because Ga atoms are autodoped from substrates and activated as donors [3,4]. Either HgCdTe(100) [5,6] or HgCdTe(111)B [7,8] epitaxial layers can be grown on GaAs(100) substrates because of the large lattice mismatch. Since the incorporated Ga concentration in HgCdTe(100) layers is lower than that in HgCdTe(111)B layers [4], HgCdTe(100) layers are attractive for preparing high purity epitaxial layers. We studied the mechanism of Ga incorporation and reduced autodoped Ga concentrations in MOVPE-grown HgCdTe(100) layers. In addition, we fabricated photodiodes using HgCdTe(100) layers with low donor concentrations, and studied the relationship between their characteristics and carrier concentrations.

partial pressure conditions. The HgCdTe growth rate was 2.5 /zm/h. HgCdTe layers were postgrowth annealed at 250°C in the Hg ambiance for 24 hours to cancel Hg vacancies, which were created during growth and became acceptors in epitaxial layers. We evaluated the carrier concentration of HgCdTe layers from Van der Pauw-Hall effect measurements at 77 K and Ga atom concentrations in CdTe and HgCdTe layers by secondary ion mass spectrometry (SIMS). SIMS measurements were done by Charles Evans and Associates. The Hg~_xCdxTe layer composition (xvalue), which we determined by room-temperature infrared transmission, was x = 0.18-0.25.

3. Results and discussion 2. Experiments

3.1. Reduction of Ga autodoping We grew epitaxial layers in a horizontal reactor with multiple nozzles and a rotating graphite susceptor heated by RF induction [9]. We used 3 inch GaAs(100) substrates misoriented 2° toward the nearest (110). We covered the back surfaces of some substrates with 100 nm thick S i 3 N 4. To improve the compositional uniformity, we injected the three precursors, dimethyl-cadmium (DMCd), diisopropyl-telluride (DIPTe), and elemental mercury (Hg), into the reactor from different nozzles. We preheated the GaAs substrates at 580°C for 20 minutes, and grew the CdTe buffer layers at 410°C under low pressure (150 Torr) to improve the thickness uniformity. To form the CdTe(100) layer, we first supplied diethyl-zinc (DEZn) and DIPTe and grew a 15 nm thick ZnTe(100) interracial layer [10] before introducing DMCd and DIPTe. We deposited the 8 /zm thick CdTe buffer layers using a growth rate of 3 /.~m/h to reduce dislocation density [2] and suppress the impurity diffusion from substrates. We grew the HgCdTe layers at 360°C under atmospheric pressure by a direct alloy growth (DAG) process [11,12]. After growing the CdTe(100) buffers, we simultaneously supplied DMCd, DIPTe, and Hg and deposited 15 to 20 tzm thick HgCdTe(100) layers under several Hg

Ga atoms were detected not in CdTe buffer layers, but in HgCdTe layers, from SIMS depth profiles (Fig. 1). This HgCdTe/CdTe layer was grown on bare GaAs substrates. The Ga ion intensity fell to the detection limit rapidly at the HgCdTe/CdTe interface. Arsenic atoms were not detected in both CdTe and HgCdTe layers. 8/zm

E

,-:,

1018

106 = 0=

I HgCdTeI

O

Cd

e- 1 0 1 7 O

105

= m

1016

eL ~ - ~U --T

10 4 "~

0 O C 1015 o 0 .~ 1014

Hg Ga

103 .O >, O~ 102 "1~ 1-

E 0 1013 0

"~

I

I

I

I

5

10

15

20

01 O O

2s

Depth (pm) Fig. 1. Typical SIMS depth profile of undoped HgCdTe/CdTe layers on GaAs bare substrates. Autodoped Ga is detected only in HgCdTe. The Ga concentration in CdTe is at the detection limit level.

H. Nishino et al. /Journal of Crystal Growth 146 (1995) 619-623

thick CdTe was thick enough to prevent Ga diffusion from the GaAs substrates. Hence, Ga atoms were incorporated from the vapor phase during HgCdTe growth. Since the HgCdTe layers carrier concentration in this sample was between 2 X 1015 and 3 x 1015 cm -3, incorporated Ga atoms were fully activated as donor impurities and dominated the transport characteristics of undoped epitaxial layers. We assumed that the back surface of GaAs substrates reacted with Hg and metalorganic precursors during HgCdTe growth and this reaction produced very volatile trimethyl-gallium (TMGa)0 TMGa evaporated into the vapor phase and acted as source of Ga autodoping. Korenstein et al. proposed similar mechanisms [4]: 3DMCd + 2Ga(GaAs) ~ 2TMGa + 3Cd,

(1)

DMCd + Hg ~ DMHg + Cd,

(2)

3DMHg + 2Ga(GaAs) ~ 2TMGa + 3Hg.

(3)

They suggested that reactions (2) and (3) were the most probable. In our experiments, reaction (1) did not occur because Ga was not detected in the CdTe layers. DMCd decomposed sufficiently at 360 to 410°C before reacting with the GaAs surface. To confirm this Ga incorporation mechanism, we coated the GaAs back surface with Si3N 4. We also investigated the dependence of Hg partial pressure on carrier concentration and Ga concentration. HgCdTe layers with carrier concentrations lower than 1015 cm -3 were obtained reproducibly on back-coated substrates (Table 1). We realized carrier concentrations between 3 x 1014 and 4 x 1014 cm -3 under low Hg partial pressure conditions. SIMS evaluation showed that this reduction was due to suppression of the Ga incorporation (Fig. 2). Comparing samples MG04 and

1019

I

E 1018

O

I

~

•L~¢- 10 16 o 1015

cm-3~

Si3N4 coated

~

C

(3

I

Uncoated

r-O 1017

0

621

1014

I 5

1013

0

I 10

cm 3_ I 15

Fig. 2. SIMS depth profile of HgCdTe on back-coated and uncoated GaAs. The autodoped Ga concentration is reduced on back-coated GaAs with Si3N 4. Ga concentrations are approximately equal to carrier concentrations in each layer.

MG10 (or samples MG01 and MG07), it is clear that the concentration of residual carriers and Ga atoms was reduced as the Hg partial pressures decreased. The fact that Hg enhances Ga incorporation is a strong evidence for the existence of reactions (2) and (3). Korenstein et al. also reported carrier concentrations on back-coated GaAs substrates [4], but their best result was n = 1.5× 1016 cm -3. This difference was explained by growth orientation and Hg partial pressure. They only grew HgCdTe(lll)B layers on back-coated substrates and possibly used high Hg partial pressures.

3.2. Characteristics of photodiodes To clarify the importance of contamination reduction in HgCdTe layers grown on GaAs by MOVPE, we demonstrated the performance of

Table 1 Electrical transport properties of MOVPE-grown HgCdTe layers under various growth conditions Layer index

Substrate back coating

Hg partial pressure (atm)

Carrier concentration (cm -3)

Electron mobility (cmZ/V • s)

MG01 MG04 MG07 MG10

none Si3N 4 none Si3N 4

3x 3x 1× 1x

3× 6x 1x 3x

6x 4× 2x 1x

10 -2 10 -2 10 -2 10 -2

20

HgCdTe depth (pm)

1015 1014 1015 1014

104 l04 105 105

Ga concentration (cm -3) 7 X 1014

3 X 10 ~5 < 2 X 1014

H. Nishino et aL / Journal of Crystal Growth 146 (1995) 619-623

622

LWIR photodiodes. We constructed n - p junctions by implanting boron ions (B ÷) into Hg vacancy-doped p-type layers. We controlled hole concentrations to p = ( 1 - 2 ) × 1016 cm -3 by post-growth annealing. The B ÷ sheet concentration in n-type regions was 1 × 1014 cm -2 and the implant energy was 140 keV. The diode area was 3.8 × 10 -5 cm 2. In this fabrication process, interstitial Hg was generated in implant regions by ion bombardment damage. Hg diffused into p-type regions during post-implantation annealing and canceled Hg vacancies [13,14]. This process changed junction profiles from n+-p to n + - n - - p structures and the carrier concentrations of the n--type regions was dominated by residual donor concentrations in epitaxial layers. Increasing n - carrier concentrations increased band-to-band tunneling current at reverse bias and it became a main component of the photodiodes' dark current. Also, the differential resistance of the photodiodes decreased at reverse bias as carrier concentrations due to residual donors in epitaxial layers increased. Our simulation showed that LWIR photodiodes with differential resistances over 1 Mr1 around the reverse bias voltage of 200 mV require residual donor concentrations below 1015 cm -3 for Hg0.8Cd0.2Te layers (Fig. 3). We evaluated LWlR photodiodes fabricated

_>,

0.3

E

=

A'°' 103

.

"~ 0.2

102

0.1

101

0.0

10° ¢

(J

-0.1 a

10"1

-400-300-200-100

0

100

Diode voltage (mV) Fig. 4. Typical current-voltage characteristics of photodiodes fabricated using MOVPE-grown HgCdTe layers on GaAs coated with Si3N 4.

using HgCdTe layers with residual donor concentrations of 4 × 1014 cm -3 grown on back-coated GaAs substrates by MOVPE. The current-voltage characteristics showed that the dark current near zero bias was limited by diffusion current and differential resistances of more than 1 Mf~ were obtained at reverse bias (Fig. 4). Typical differential resistance-area products at zero bias (RoA) and maximum differential resistance-area products (Rd(max)A) were 1 and 750 ~ . cm 2 at 77

Wavelength (pm) 105

....,0 101o

o=

I

¢~ 108 .~

ira=

104

..,

bias

\

",,

Eg=O.124eV ', ~ - N a = l x l O l s c m "3 " , , \ Vd = 0.04 V ,%

In

~.. 102 1014 C3

I 1015

(mV) ~

50

~100

(3

10

9

8

x,V.

,everse

'

",, \,

a) 106 .~ "~

\

2 11

10 3

0 "~"0 101 n-

~=~ 200

" ~ ' ~-- 400 1016

n- carrier concentration (cm -z) Fig. 3. Calculated differential resistance of LWlR photodiodes under various reverse bias conditions. The resistance decreases with n - carrier (residual donor) concentration because the band tunneling current increases.

10"1

,

0.10

,

0.12

i

L

,

0.14

0.16

Energy gap (eV) Fig. 5. Differential resistance-area product of HgCdTe LWIR photodiodes prepared by different growth methods. Data of LPE-HgCdTe/CdZnTe and bulk-HgCdTe are taken from Refs. [15-18]. HgCdTe layers: (©, e) MOVPE on GaAs; 0% II) LPE on CdZnTe; (o, 0 ) bulk HgCdTe.

H. Nishino et aL / Journal of Crystal Growth 146 (1995) 619-623 K with a cutoff wavelength of 10.9 ~m. We compared R o A and Ra(max)A of our M O V P E grown H g C d T e / G a A s photodiodes with LPEgrown H g C d T e / C d Z n T e [15,16] or bulk HgCdTe [17,18] photodiodes for cutoff wavelengths of 9 to 12 /zm (Fig. 5). Destefanis and Chamonal [16] also obtained R o A of LPE-diodes one order of magnitude larger than that in Fig. 5 using their new "modified process". We compared our MOVPE-diodes with their LPE-diodes fabricated with their "standard process", which was similar to ours, in order to evaluate epitaxial layer quality. The performance of MOVPE-diodes was as good as conventional LPE- or bulk-diodes, hence, we confirmed that reducing autodoped Ga concentrations during M O V P E growth was important for fabricating L W I R photodiodes on GaAs substrates.

4. Summary We studied the mechanism of Ga autodoping in HgCdTe layers grown on GaAs by M O V P E and determined growth conditions suitable for reducing the carrier concentrations in undoped HgCdTe layers. Since Ga atoms were detected only in HgCdTe layers, Ga atoms did not diffuse into HgCdTe layers from GaAs substrates through CdTe buffer layers, but were incorporated from the vapor phase during H g C d T e growth. We proposed the following mechanism of Ga incorporation: The GaAs back surface reacted with Hg and metalorganic precursors during HgCdTe growth and this reaction produced volatile T M G a which acted as the Ga autodoping source. To confirm this model, we coated the GaAs back surface with Si3N 4 and obtained HgCdTe layers with carrier concentrations below 1015 cm -3. SIMS evaluation showed a decreased carrier concentration due to suppression of Ga incorporation. We also clarified that Hg enhanced Ga incorporation. We realized MOVPE-grown HgCdTe layers with carrier concentrations between 3 × 10 t4 and 4 × 1014 cm -3 on back-coated GaAs substrates under low Hg partial pressure. Using this low contamination epitaxial layer, we fabricated L W l R photodiodes whose performance was as

623

good as those fabricated using LPE-grown H g C d T e / C d Z n T e layers. Theoretically and experimentally, we confirmed that reducing Ga autodoping is necessary to suppress band-to-band tunneling current, which degrades diode performance.

Acknowledgments We thank Dr. H. Takigawa for his useful advice. We also thank Dr. H. Ishizaki and Dr. S. Yamakoshi for their encouragement.

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