Ohmic contacts to n-type and p-type ZnSe

PERGAMON

Solid-State Electronics 43 (1999) 113±121

Ohmic contacts to n-type and p-type ZnSe M.R. Park a, W.A. Anderson a, *, M. Jeon b, H. Luo c a

Department of Electrical and Computer Engineering, State University of New York at Bu€alo, 217 Bonner Hall, Amherst, NY 14260, USA b Photonics Lab., Materials Sector, Samsung Advanced Institute of Technology, PO Box 111, Suwon 440-600, South Korea c Department of Physics, State University of New York at Bu€alo, 126 Fronczak, Amherst, NY 14260, USA Received 30 March 1998; accepted 20 May 1998

Abstract Stable Ohmic contacts are essential for reliable operation of electronic devices. Such contacts have been made to n-type and p-type ZnSe. Au, Pd, Cu and Se for p-type N-doped ZnSe (1  1017 cm ÿ 3) and AuGe, In, Yb and Mg for n-type Cl-doped ZnSe (4.5  1018 and 1.15  1019 cm ÿ 3) grown by molecular beam epitaxy (MBE) on (100) semi-insulating GaAs substrates have been deposited by thermal evaporation. Annealing techniques at di€erent temperatures, chemical etching and cleaning prior to metallization and reactive ion etching (RIE) in a N2 plasma and a Ar plasma for p-ZnSe have been studied. The electrical characteristics for the contacts were examined by the current versus voltage curves and the speci®c contact resistance was determined by use of the transmission line method (TLM). The current transport mechanisms for the Mg/Au contact to n-type ZnSe and the Cu/Au contact to p-type ZnSe have been studied by the current versus voltage for di€erent temperatures (I±V±T) measurements. In/ Au was best for n-type and Cu/Au for the p-type materials. Plasma treatment of the ZnSe surface prior to metallization was proven to lower the contact resistance to p-type ZnSe. The lowest speci®c contact resistance values of 1.67  10 ÿ 1 O cm2 for the Cu/Au contact to p-type ZnSe with a N2 plasma treatment and 1.04  10 ÿ 2 O cm2 for the In/Au contact to n-type ZnSe were achieved. Two di€erent current ¯ow mechanisms were shown for the Cu/Au contact to low doped p-ZnSe (1  1017 cm ÿ 3) and three for Mg/Au contact to highly doped n-ZnSe (1.15  1019 cm ÿ 3). The Cu/Au contact to p-ZnSe and Mg/Au contact to n-ZnSe have been observed to be especially stable and reproducible. # 1998 Published by Elsevier Science Ltd. All rights reserved.

1. Introduction The wide-bandgap II±VI compound semiconductor zinc selenide(ZnSe) is regarded as a promising material for optoelectronic devices. Due to a bandgap of 2.7 eV at room temperature, ZnSe is a potentially good material for blue±green laser diodes. The formation of low resistance Ohmic contacts is important for the successful operation of devices since this allows lower operating voltage and minimizes dissipation of power. Since the mechanism of Ohmic contact formation to ZnSe, ZnS and mixed crystal ZnSxSex ÿ 1 was devel-

* Corresponding author. Tel.: +1-716-645-2422; Fax: +1716-645-3656.

oped by Gilbert [1]. In metal was well known as the ohmic contact to n-type ZnSe with a speci®c contact resistance of 5  10 ÿ 2 O cm2 [2]. However, this metal contact has low reliability due to high contact resistance and poor wetting. Although a Ti/Pt/Au multilayered metal structure showed good Ohmic behavior for n-type ZnSe with an electron concentration of 2  1019 cm ÿ 3 and a speci®c contact resistance of 3.4  10 ÿ 4 O cm2 [3], a single metal has not successfully formed an ohmic contact to n-ZnSe. Furthermore, it is more dicult to obtain good Ohmic contacts to p-ZnSe since the work function of ZnSe is very high compared to the work function of metals. Recently, more work has been done with Ohmic contacts to p-type ZnSe. Fijol et al. [4] reported that the reverse bias breakdown voltages of the Ag contact

0038-1101/98/$ - see front matter # 1998 Published by Elsevier Science Ltd. All rights reserved. PII: S 0 0 3 8 - 1 1 0 1 ( 9 8 ) 0 0 1 9 1 - 9

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(MBE) on a (100) semi-insulating GaAs substrate have been studied. To form ohmic contacts, annealing techniques at di€erent temperatures, chemical etching and cleaning prior to metallization and reactive ion etching (RIE) in a N2 plasma and an Ar plasma for p-ZnSe have been studied. The speci®c contact resistance was determined by use of the transmission line method (TLM).

2. Experimental Fig. 1. Schematic of the TLM pattern.

and Au contact to p-ZnSe were reduced to 2.3 and 3 V by the heat treatment, respectively. Rennie [5] discussed the in¯uence of a sulphur-based treatment and a CuSex contact treatment prior to the deposition of Pd/Au metallic contact to p-type ZnSe. The electrical properties and thermal stability of a variety of metals (In, Cd, Nd, W, Cu, Ag, Au, Ni, Pt, and Se) contacts to p-type ZnSe were reported by Tadanaga [6]. In this study, a chemical etching treatment was used to improve the electrical properties. In this paper, Au, Pd, Cu and Se contacts to p-type N-doped ZnSe(1  1017 cm ÿ 3) and AuGe, In, Yb and Mg contacts to n-type Cl-doped ZnSe (4.5  1018 and 1.15  1019 cm ÿ 3) grown by molecular beam epitaxy

The N-doped, p-type and Cl-doped, n-type ZnSe were epitaxially grown by molecular beam epitaxy (MBE) on (100)-oriented semi-insulating GaAs substrates. The carrier concentrations of p-type ZnSe layers and n-type ZnSe layers were 1  1017 cm ÿ 3 and in the range of 4.5  1018 to 1.15  1019 cm ÿ 3, respectively. The thickness of ZnSe layers was between 0.7 to 1 mm. Prior to chemical cleaning, some of the p-type samples were treated with a N2 plasma and on Ar plasma in a reactive ion etching (RIE) system. The cleaned samples were patterned by a two step photoresist process to form the mesa structure and the metal contact pads which conform to the transmission line method (TLM) for determining speci®c contact resistance. This pattern has ®ve square contact pads, 50  50 mm, spaced at four distances, 50, 100, 150 and

Fig. 2. In/Au contacts to n-ZnSe doped at (a) 4.5  1018 cm ÿ 3 and (b) 1.15  1019 cm ÿ 3.

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115

Fig. 3. Mg/Au contact to n-ZnSe doped at 1.15  1019 cm ÿ 3: (a) I±V data and (b) transmission line method data.

200 mm(Fig. 1). The mesa structure was achieved with a solution of 1 g K2Cr2O7: 10 ml H2SO4: 20 ml D.I. water for 3±7 min. To remove the native oxide of the ZnSe layer and clean the surface, a solution of HBr and H2O was used right before the metal deposition. All of the metal was evaporated with a thickness of 500±800 AÊ and at a vacuum of 1.5  10 ÿ 6±3  10 ÿ 6 Torr. After the metal deposition, the samples were heat treated in a furnace ®lled with an ambient of forming gas (10% hydrogen and 90% nitrogen) at temperatures of 200, 250, 300 and 3508C. Between each heat treatment interval, the current±voltage (I±V) characteristic of each contact on the sample was measured by a two-point probe method and the end resistance and the total resistance for calculating speci®c contact resistance were measured by a fourpoint probe method. 3. Experimental results 3.1. n-type ZnSe The current versus voltage curves for as-deposited metal and heat treated In/Au contacts to n-ZnSe are shown in Fig. 2. For the moderately-doped n-ZnSe (4.5  1018 cm ÿ 3 ) the contact seems to be rectifying rather than Ohmic and a reduction of the reverse bias break-over voltage by a factor of 02.5 V was obtained

by heat treatment over 2508C (Fig. 2(a)). In contrast, the sample with high doping concentration shows an almost linear current vs. voltage curve as deposited with a speci®c contact resistance of 1.04  10 ÿ 2 O cm2 in the voltage range of 1.5±10 V without heat treatment (Fig. 2(b)). The I±V characteristics of the In/Au contacts were not reproducible and were unstable during measurements at room temperature. Fig. 3(a) and (b) show the I±V curves and the total resistance versus the spacing between contacts of the Mg/Au contact to n-ZnSe with an electron concentration of 1.15  1019 cm ÿ 3. The resistance values of the Mg/Au contact gradually increase with increasing annealing temperatures. The speci®c contact resistance by the TLM method was 9.95  10 ÿ 2 O cm2, which is slightly higher, but of the same order as the In contact. However, in comparison to the In contact, the resistance values of the Mg contact are almost constant in the whole voltage range and the electrical properties are stable and reliable. AuGe, Au and Yb contacts to n-ZnSe form a rectifying contact, although those contacts have low resistance values in the linear region of the I±V curve. The resistance di€erences between the linear and non-linear regions of the I±V curves are too large to be an Ohmic contact. The I±V characteristics of these contacts to nZnSe after heat treatment agree with the In and Mg contacts to n-ZnSe for the sample with the di€erent doping concentrations.

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Fig. 4. Se/Au contacts to p-ZnSe doped at 1  1017 cm ÿ 3: (a) annealed at various temperatures for 15 min and (b) without N2 plasma treatment and with N2 plasma and Ar plasma treatment.

3.2. p-type ZnSe I±V characteristics for Se/Au contacts to p-ZnSe are shown in Fig. 4 for di€erent annealing temperatures without N2 plasma treatment (Fig. 4(a)) and di€erent treatment times using a N2 plasma (Fig. 4(b)) before the sample cleaning. For contact formation without additional N2 plasma treatment, heat treatment at 3008C caused a lower break-over voltage of 010 V than for the as-deposited case. Heat treatment above 3508C resulted in an increase in the break-over voltage and the resistance. From the e€ect of plasma treatment, we can see that the sample which was N2 plasma treated for 10 min and annealed at 3008C had a better response than the untreated sample over the voltage range of 0±10 V. From a comparison of the I±V characteristic for N2 plasma treated and Ar plasma treated samples (Fig. 4(b)), N2 treatment caused more current ¯ow than for the untreated sample. Cu/Au contacts to p-ZnSe show lower Ohmic contact resistance in the whole voltage range. The I±V curves for N2-treated and untreated Cu/Au contacts are shown in Fig. 5(a). Note that the N2 plasma treated sample has better Ohmic behavior and a lower contact resistance value of 1.67  10 ÿ 1 O cm ÿ 2, which is one order less than that of the untreated sample. However, the Cu/Au metal contact to p-ZnSe has

quite di€erent electrical characteristics compared to other metal contacts. From Fig. 5(b) we can see that heat treatment over 2508C reduced the contact resistance. This result indicates that annealed contact properties are similar to Au contacts due to the di€usion of Au into Cu as heated. In contrast to other metal contacts, Au into Cu contacts have stable and consistent properties for di€erent contacts on the same sample. I±V characteristics degraded somewhat a week after the contact was made and then became stable. Using a Br etching solution prior to metallization reported by Ando [7] did not produce any measurable improvement but instead reacted with the photoresist and caused failure during pattering of the sample. Pd and Au contacts gave Schottky behavior with high resistance and reverse break-over voltages of 11 and 18 V, respectively. All metal contacts to n-type and p-type ZnSe have symmetric behavior of I±V curves, which is typical for the back to back diode, in the positive and negative voltage ranges. 3.3. I±V characteristics with temperature The I±V characteristics for di€erent temperatures show that the Cu/Au contact to p-ZnSe (Fig. 6) and the Mg/Au contact to n-ZnSe (Fig. 7) have di€erent current transport mechanisms. Fig. 6(a) for Cu/Au to

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Fig. 5. Cu/Au contacts to p-ZnSe doped at 1  1017 cm ÿ 3: (a) with and without plasma treatment and (b) dependence on the heat treatment and time after deposition.

p-ZnSe shows temperature to have a greater e€ect at higher voltage where Fig. 7(a) for Mg/Au to n-ZnSe shows mainly temperature independence at higher voltages. The currents increase with temperature so the resistance decreases in the whole regime of the voltage for the Cu/Au contact to p-ZnSe, but much less for the Mg/Au contact to n-ZnSe, where it becomes constant above 9.5 V. At that point, the mechanism changes as evidenced by a change in slope. The Mg/Au contact to n-ZnSe has a lower resistance than the Cu/Au contact to p-ZnSe. The estimated barrier height values (FB) were determined from log(I) vs. V plots and Eq. (1), ignoring Schottky barrier lowering, to be 0.53 and 0.50 eV for the Cu/Au and Mg/Au contacts, respectively. The parameters used for the calculation were A **/A = 0.62 (A = 120 A/cm2/K2) and n = 1. I ˆ SA** T 2 eÿqFB=kT …eqV=nKT ÿ 1†

…1†

The thermionic emission current is given by Ref. [8]. Then, the tunneling current is given by Ref. [8]. s q h N I ˆ S exp…ÿqFB=Eoo †, Eoo ˆ …2† 2 es m * where S is the contact area, n is the ideality factor, A is Richardson's constant, N is doping concentration and m * is the tunneling e€ective mass.

4. Discussion For n-type ZnSe, the In/Au contact has the lowest value of speci®c contact resistance (1.04  10 ÿ 2 O cm2) but unstable characteristics. Although the speci®c contact resistance of Mg/Au contacts is slightly higher (9.95  10 ÿ 2 O cm2) than that of the In/Au contact, those values are stable in the whole range of applied voltage and are reproducible. The higher doped sample has better Ohmic behavior than the low doped sample and requires no heat treatment. This is due to a tunneling of electrons through the potential barrier with increasing carrier concentration. For p-type ZnSe, the lowest contact resistance value of 1.67  10 ÿ 1 O cm2 was achieved by the Cu/Au contact, N2 plasma pretreated for 10 min and not heated. The Cu/Au contact has better Ohmic behavior and stability than the Se/Au contact even though Cu(4.7 eV) has a lower work function than Se(5.9 eV). N2 plasma treatment causes two di€erent e€ects; lowering of the break-over voltage and little change in resistance value for the Se/Au contact and the reverse for the Cu/Au contact. Plasma processing of p-ZnSe prior to metallization can create defect states within the tunneling barrier, leading to a reduced contact resistance [9]. Negative e€ects are also possible such as compensation of surface doping or impurity gettering. The calculated value of the speci®c contact resistance from the thermionic emission current equations is

Fig. 6. Cu/Au contact to p-ZnSe with plasma pre-treatment: (a) I±V plot at various temperatures, (b) log(I) vs. V plot, and (c) the resistance variation with temperature.

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Fig. 7. Mg/Au contact to n-ZnSe: (a) I±V plot at various temperatures, (b) log(I) vs. V plot, and (c) the resistance variation with temperature.

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Table 1 Properties of the various metal contacts on n-type and p-type ZnSe Semiconductor

Deposited metal

type

doping (cm ÿ 3) concentration

n n n p p

1.15  1019 4.5  1018 1.15  1019 1  1017 1  1017

In/Au In/Au Mg/Au Se/Au Se/Au

p p p

1  1017 1  1017 1  1017

Cu/Au Cu/Au Cu/Au

Special treatment

N2 plasma for 10 min N2 plasma for 10 min bromine etching

Conditions

Reverse bias break-over voltage (V)

Resistance (O)

Speci®c contact resistivity (O cm2)

As deposited 2508C, 10 min As deposited 3008C, 15 min As deposited

1.5 10

416 (1.5±10 V) 47.2  103 (13±15 V) 0.7  103 (3.1±5 V) 135.8  103 (10±15.5 V) 687.3  103 (19±20 V)

1.04  10 ÿ 2 1.18 9.95  10 ÿ 2 3.40 17.18

As deposited As deposited As deposited

0.205 O cm2 (the estimated barrier height = 0.53 eV) for the Cu/Au contact to p-ZnSe with N2 reactive ion treatment. This value is consistent with the measured value of 0.167 O cm2 (Table 1). But, for the Mg/Au contact to n-ZnSe, the calculated value of 0.963 O cm2 (the estimated barrier height = 0.50 eV) does not agree with the measured value of 9.95  10 ÿ 2 O cm2 (Table 1). On the other hand, the values of the speci®c contact resistance from the tunneling current equations

10 19

10.9  103 (0±11 V) 44.0  103(0±14 V) 93.5  103 (0±25 V)

1.97 1.67  10 ÿ 1 2.34

are much lower than the measured values for both contacts. Two di€erent current ¯ow mechanisms are involved for the Cu/Au contact and three for the Mg/Au contact. Fig. 6(a) shows a crossing of some of the constant temperature lines. This has been observed on several samples and may be due to plasma-induced defects. Data from temperature testing in Fig. 6 were also analyzed using the equation

Fig. 8. Log(I) vs. T plot: (a) Cu/Au contact to p-ZnSe with plasma pre-treatment and (b) Mg/Au contact to n-ZnSe.

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Table 2 Evaluation of Fig. 6 using I = Io expaV expbT Cu/Au on p-ZnSe voltage range

I0(A)

a

b

0±2.5 V 3±6.5 V 7±20 V

9.28  10ÿ9 5.57  10ÿ8 2.92  10ÿ7

1.053 0.371 0.152

1.21  10ÿ2 1.21  10ÿ2 1.21  10ÿ2

I ˆ Io expaV expbT

…3†

where a and b are constants. Table 2 summarizes the results. It is clear that the mechanisms are very similar and that voltage dependence is much stronger than temperature dependence. The weak temperature dependence is shown in Fig. 8, with similar results for Cu/ Au on p to Mg/Au on n. However, temperature dependence becomes smaller as voltage increases for the Mg/ Au contact even though it is constant in the whole voltage range for the Cu/Au contact. Thus, it seems that thermionic emission current dominates in the low voltage region and other current mechanisms become involved as voltage increases such that thermionic ®eld emission or tunneling prevail over much of the higher voltage region.

5. Conclusions The thermionic emission transport mechanism dominates in the low voltage region and thermionic ®eld mechanism in the higher voltage region for low and highly doped ZnSe. However, the mechanism of contact to highly doped n-ZnSe is ®eld emission. For the contact to p-ZnSe, N2 plasma pre-treatment plays an important role of decreasing the contact resistance but mechanisms are not well known. The Ohmic contact formation to ZnSe is dependent on the doping concentration of the ZnSe layer and the reactivity and ad-

hesion of the metal with the surface of ZnSe rather than the work function of the metal. Acknowledgements The authors would like to thank E. H. Lee, H. C. Cheng and M. H. Na for providing ZnSe samples. References [1] Kaufman RG, Dowbor P. J Appl Phys 1974;45:4487. [2] Jeon H, Ding J, Nurmikko AV, Xie W, Grillo DC, Kobayashi M, Gunshor L, Hua GC, Otsuka N. Appl Phys Lett 1992;60:2045. [3] Miyajima T, Okuyama H, Akimoto K. Jpn J Appl Phys 1992;31:L1743. [4] Fijol JJ, Calhoun LC, Park RM, Holloway PH. J Electron Mater 1995;24:143. [5] Rennie J, Onomura M, Nishikawa Y, Saito S, Ishikawa M, Hatakoshi G. Jpn J Appl Phys 1996;35:1664. [6] Tadanaga O, Koide Y, Hashimoto K, Oku T, Teraguchi N, Tomomura Y, Suzuki A, Murakami M. Jpn J Appl Phys 1996;35:1657. [7] Ando K, Hirakawa T, Ohki A, Ohno T, Yonezawa H. Ext. Abstr., 1995 Int. Conf. Solid State Devices and Materials, Osaka, 1995:680. [8] Sze SM. Physics of semiconductor devices. New Jersey: John Wiley and Sons, 1981:304. [9] Fahrenbruch AL, Bube RH. Fundamentals of solar cells. Academic Press, 1983:187±209.