3-3-1 XPS and PL of the sulfided GaAs substrate
The surface treatment of GaAs in sulfur solution results in a dramatic decrease surface state density in the middle of the band gap. Reduction of surface recombination velocity enables an improvement in the performance and reliability of many devices. For the beginning, three sulfur passivation methods were used to increase the efficiency on HCl-cleaned GaAs substrate, which was shown in the Figure 3-1.
First, Sodium sulfide is the salt of a strong base and a week acid, after its dissolution in D.I.W.:
2 2 2
Na S +H O U Na+ + HS− +OH− (1)
Second, Ammonium sulfide is the salt of a week base and a weak acid. In the D.I.W. reaction occurs:
2
4 3 4 3 2
2NH + + S − U NH + NH + + HS− U 2NH + H S (2)
Both above sulfur coat formation can occur either by reaction (3) [37].
2 2
GaAs
+H S
UGaS
+AsS
+H
↑ (3)32
Finally, the organic twelve-carbon thiol with long-chain molecule type was used. The molecule was sulfur-terminated molecule which can be bonded with gallium or arsenic element of substrate. Fig. 3-1 shows the set of (a) As 2p and (b) Ga 2p core-level spectra after three sulfide treatment. The substrate peaks were normalized to the same height and substrate signals shifted to the same binding energy which was As(GaAs):1323 ± 0.5eV; Ga(GaAs):1117.5 ± 0.5eV. This study has taken a step in the direction of defining the relationship between sulfur and GaAs surface passivation in order to suppress the oxides.
The sulfur signal also can be seen from the S 2p orbit spectra, as shown in the Fig. 3-2, and Fig. 3-2(a) points out the sulfur peak, which is overlap with Ga 3s peak, and the spacing about 1.1eV. Fig.3-2 (b) is the subtraction of S 2p and Ga 3s orbit, and we can subtract Ga 2s peak of no-sulfided sample from S 2p peak of sulfided one. The S 2p orbit can see the sulfur peak signal, but we don’t like to use this indirect way. The suppression of sulfided samples after 1week compared to no-sulfided one is shown in Fig. 3-3. Clearly, the sulfided sample can decrease the growth rate compared to no-sulfided one. Hence, the sulfur passivation for GaAs substrate is useful and further research is required. There are two sulfur methods with PL analysis as shown in Fig. 3-4 and Fig. 3-5. The Fig. 3-4(a) is the Photoluminescence(PL) intensity treated by (NH4)2S/D.I.W.
33
liquid. It shows that sulfided temperature increasing the PL intensity before 400
oC, and it decreases at 500 oC. This is because arsenic bonds fully broke before
400 oC, and Ga compound bonds began to desorb. Then, the solvent effect was replaced with C3H7OH solution, whose results were shown in Fig.3-4(b). When anneal at 400oC and highest concentration with 10% will obtain the highest PL intensity. The highest PL intensity cause may be the more sulfur bond coverage.
The Fig. 3-5 is the Photoluminescence(PL) intensity treated by Na2S/D.I.W.
liquid . Fig. 3-5(a) shows that sulfided temperature increasing the PL intensity at 400oC, and it decrease at 500 oC. Then, the solvent was chosen with C3H7OH solution, whose results were shown in Fig.3-5(b). When the PDA anneal at 400
oC, highest concentration with 2.5% will increase PL intensity.
Fig. 3-6(a) and (b) illustrate XPS spectra of the sulfide temperature effect.
When the temperature is increasing, the Ga-S bonds will increase and As-S bonds decrease. This is because that thermal stability of Ga-S bonds is better than As-S bonds. Fig.3-7 shows different ratio of GaAs surface signal, and we divide into five parts: (i) As element (ii) As-S (iii) Ga-S (iv) As-O (v) Ga-O.
After sulfidation, the As element decrease from 13.9% to 3.5%, and oxides decrease one half at least. About sulfur composition, the high sulfided temperature causes the Ga-S bonds increasing compared to As-S bonds
34
decreasing. Fig.3-8 (a) and (b) are spectra of GaAs with two sulfur concentration, we believe that sulfur really suppresses the GaAs oxide under 80oC passivation solution. Fig. 3-9 (a) and (b) are sulfur concentration at Ga orbit spectra. They also show the same behavior compared to Fig. 3-8. In the Fig. 3-10(a) and (b), we discuss the alkaline NH4OH solution effect combining with sulfur passivation on the GaAs surface. From the Fig. 3-10 (a), we find the As element peak magnitude of (i) and (ii) are different. With NH4OH solution cleaned, the As element component will decrease, because of As(OH) bonds can remove easily compared to Ga(OH) bonds. When sulfided temperature increase, the As-O and As element both expressed decreasing behavior. Fig. 3-10 (b) is the Ga 2p spectrum, we find that the Ga-S peak increase with 80oC sulfur temperature compared to RT treatment. The Ga element likes to react with sulfur solution at high temperature. Simply stated, the NH4OH solution etching can reduce As compound bonds and get a Ga-termined surface .Fig. 3-11 shows different ratio of GaAs surface signal, and we divide into five parts: (i) As element (ii) As-S (iii) Ga-S (iv) As-O (v) Ga-O. After NH4OH and sulfur solution, the As element decrease from 9.6% to 4.7%, and oxides also decrease.
About sulfur composition, the high sulfide temperature cause the Ga-S bonds increase .On the other hand, As-S bonds decrease with high sulfided
35
temperature.
3-3-2 surface roughness of the sulfided GaAs
Fig. 3-12 shows the variation of surface roughness of GaAs after the cleaning of diluted HCl/D.I.W. (10%) etching solution with three sulfide conditions of (a) without (b) 1%, RT (c) 1%, 80oC.The surface roughness decrease as sulfide temperature increase from 0.354nm to 0.237nm. Fig. 3-13 shows the variation of surface roughness of GaAs after the cleaning of diluted HCl/D.I.W.(10%) and NH4OH/D.I.W.(1%) etching solution with two sulfide conditions of (a) without (b) 1%, 80oC. The surface roughness ranges from 0.265nm to 0.235nm. Fig. 3-14 shows the variation of surface roughness of GaAs after the cleaning of diluted NH4OH/D.I.W.(1%) etching solution with two sulfide conditions of (a) without (b) 1%, 80oC. The surface roughness ranges from 0.266nm to 0.245nm. All the variation of surface roughness after different treatment is summarized in Fig.3-15, and the GaAs surface roughness is smooth with different surface treatment.
36
3-3-2 electrical characterization of the sulfided GaAs
In Fig. 3-16, we treatment the HCl cleaned GaAs substrate with two sulfur solutions combined with three solvents. The curves were measured at room temperature, and we find that the I-V curves depend on surface treatment. As displayed in Fig. 3-17 was the extraction of Schottky barrier height(SBH) by following equation:
(1)
* 2
Where J =A T e
s B qkT
− Φ
, A* is the Richardson constant; n is the ideal factor; ΦBis the barrier height. Fig. 3-18 and 3-19 show the variation of barrier height with different sulfide treatment. Because that the ideal barrier height difference of Al metal contact to n-GaAs substrate is about 0.4 eV, and the barrier height of sulfided GaAs is smaller than no-sulfidation one. From the Fig. 3-19, the barrier height decreased with decreasing dielectric constant of solvent solution. This is because that efficiency for different solvents solution. Fig. 3-20 shows I-V curves difference before and after sulfidation, and we find that original Schottky-like diode curve has change to ohmic-like curve.
-qV/nkT
J=J (1-e
s)
37
3-4 Summary
After sulfur passivation, the GaAs surface elements bond with sulfur element. The formation of As-S or Ga-S bond depends on clean method and sulfided temperature. The HCl etching solution remains the As-terminated surface, but NH4OH remains the Ga-terminated surface. We can use the sulfided temperature to mathch with GaAs surface element. Because of thermal stability for As-S bonds are worse better than Ga-S bonds. We need to increase the sulfided temperature up to 80oC, and Ga-S bonds remain stable. The Schottky barrier height (SBH) can be changed to ideal-like value by sulfidation, especially in solvent solution.
37
1332 1330 1328 1326 1324 1322 1320
0.0
1122 1120 1118 1116 1114 1112
0.0
Fig. 3-1 (a) As 2p and (b) Ga 2p core-level spectra after different sulfide methods.
38
Fig. 3-2 (a) S 2p spectra by three sulfur passivation before subtraction and (b) after subtraction Ga 2s peak from S 2p peak.
170 168 166 164 162 160 158 156 154 152
As-S/Ga-S
Intensity
Binding energy (eV)
No Sulf.
(NH4)2S treat.
Na2S treat.
12C-Thiol treat.
S2p3
&Ga3s overlap
(a)
170 168 166 164 162 160 158 156 154 152
Intensity (b)
Binding energy (eV)
(NH4)2S treat.
Na2S treat.
S2p3 As-S/Ga-S
39
1332 1330 1328 1326 1324 1322 1320 1318
(a)
Photoelectron intensity (arb.units)
Binding energy (eV) w/o sulfide
w/o sulfide after 1week with sulfide
with sulfide after 1week As 2p3
As(GaAs) As-O
1122 1120 1118 1116 1114 1112
(b)
Photoelectron intensity (arb.unit)
Binding energy (eV) w/o sulfide
w/o sulfide after 1week with sulfide
with sulfide after 1week Ga 2p
3 Ga(GaAs)
Ga-O
Fig. 3-3 (a) As 2p and (b) Ga 2p core-level spectra of the sulfided samples against moisture with spectra after 1week .
40 with different (a) temperature and (b) concentration.
41
Fig. 3-5 Photoluminance of Gd2O3/ GaAs sulfided by Na2S solution with different (a) temperature and (b) concentration.
42
Fig. 3-6 (a) As 2p (b) Ga 2p core-level spectra of sulfided GaAs at various treatment temperature: (i) without sulfide (ii) sulfide at room temperature (iii) sulfide at 50oC (iv) sulfide at 80oC.
1330 1328 1326 1324 1322 1320 1318
(iii) 80OC Sulfide
1122 1120 1118 1116 1114 1112
(iii) 80oC Sulfide
43
Fig. 3-7 GaAs component ratio of the HCl cleaned with two sulfide temperature.
44
1330 1328 1326 1324 1322 1320 1318
(iii) 2% Sulfide
Fig. 3-8 (a) As 2p and (b) As 3d spectra of GaAs after two sulfur concentration:
(i) cleaned (ii) sulfide at 0.5% concentration (iii) sulfide at 2% concentration.
45
1122 1120 1118 1116 1114
(iii) 2% Sulfide
Fig. 3-9 (a) Ga 2p and (b) Ga 3d spectra of GaAs after two sulfur concentration:
(i) cleaned (ii) sulfide at 0.5% concentration (iii) sulfide at 2% concentration.
46
1330 1328 1326 1324 1322 1320 1318
(iii) HCl_NH4OH_
1122 1120 1118 1116 1114 1112
(iii) HCl_NH4OH
Fig.3-10 (a) As 2p and (b) Ga 2p core level spectra after HCl+NH4OH solution with two sulfided temperature.
47
Fig. 3-11 GaAs component ratio of the HCl、NH4OH solutions cleaned with two sulfide temperature.
48 HCl(10%)
Rms (Rq) 0.354nm HCl(10%) +Sulfide(1%,RT)
Rms (Rq) 0.296nm
HCl(10%) +Sulfide(1%,80oC)
Rms (Rq) 0.237nm (a)
(b) (c)
Fig. 3-12 Surface morphology of HCl cleaned GaAs substrate after different sulfide condition: (a) without sulfidation (b) 1%, RT (c) 1%, 80oC.
49 Rms (Rq) 0.265nm
HCl(10%) + NH4OH(1%)
HCl(10%) + NH4OH(1%) + Sulfide(1%,80oC)
Rms (Rq) 0.235nm
(a) (b)
Fig. 3-13 Surface morphology of HCl +NH4OH cleaned GaAs substrate after different sulfide condition: (a) without sulfidation (b) 1%, 80oC.
50 Rms (Rq) 0.266nm
NH4OH(1%) NH4OH(1%)+Sulfide(1%,80oC)
Rms (Rq) 0.245nm
Fig. 3-14 Surface morphology of NH4OH cleaned GaAs substrate after different sulfide condition: (a) without sulfidation (b) 1%, 80oC.
51 0.1
0.2 0.3 0.4 0.5
w/o w/o (1%,80OC)
NH4OH(1%) HCl(10%)
HCl(10%)+NH4OH(1%)
Surface roughness ,Rms (nm)
area=1umx1um
(1%,80OC) (1%,80OC)
(1%,RT)
Sulfide conditions w/o
Fig. 3-15 Surface roughness of cleaned GaAs with different sulfidation
52
Fig. 3-16 I-V curves of Al metal contact to n-GaAs substrate with different surface treatment.
Fig. 3-17 Extraction of Schottky barrier height(SBH) with Al metal on n-type GaAs
0.00 0.05 0.10 0.15 0.20
1E-7
53
Fig. 3-18 Schottky diode parameters with (NH4)2S solution after (i) no-anneal (ii) 400oC N2 anneal (iii) 500oC N2 anneal.
Fig. 3-19 Schottky diode parameters with sulfur solution and different solvents: (i) H2O (ii) C3H7OH (iii) C4H9OH.
54
Fig. 3-20 Schottky barrier height(SBH) with sulfur solution and different solvents : (i) H2O (ii) C3H7OH (iii) C4H9OH.
Fig. 3-21 I-V curves of Schottky diode (a) with and (b) without sulfide.
0.55
55