Modification of Hg1−xCdxTe and related materials by ion-beam treatment

Journal of Alloys and Compounds 371 (2004) 153–156

Modification of Hg1−x Cdx Te and related materials by ion-beam treatment K.D. Mynbaev∗ , V.I. Ivanov-Omskii Ioffe Physico-Technical Institute, Polytechnicheskaya 26, St. Petersburg 194021, Russia Received 13 October 2002; received in revised form 3 July 2003; accepted 5 July 2003

Abstract Modification of ternary Hg1−x Cdx Te alloys with 0.21 ≤ x ≤ 0.55 and quaternary Hg1−y−z Zny Cdz Te alloys by treatment with low-energy (E < 800 eV) Ar ions was studied. It is shown that for conductivity type conversion in Hg1−x Cdx Te with 0.28 ≤ x ≤ 0.39, it is essential to use a neutralised ion beam. The observed dependence of the conversion depth on x agrees with the diffusion conversion model. The effect of self-compensation of intrinsic acceptor defects by intrinsic donors during the annealing prior to ion treatment is confirmed. This effect provides the possibility of controlling the carrier concentration in the n-region of p–n junctions fabricated by ion-beam treatment in vacancy-doped Hg1−x Cdx Te. © 2003 Elsevier B.V. All rights reserved. PACS: 61.72.Vv; 61.80.Jh; 66.30.Jt; 72.80.Ey; 73.61.Ga; 79.20.Rf Keywords: Semiconductors; Point defects; Electrochemical reactions

1. Introduction The low energy of intrinsic defect formation in mercury cadmium telluride (Hg1−x Cdx Te) alloys is known to hinder using conventional techniques of semiconductor processing for this material. On the other hand, easy intrinsic defect generation makes it possible to control the electrical properties of the material solely by changing the concentration of these defects, without needing to introduce an extrinsic dopant. This can be done, for example, by low-energy (E < 2 keV) ion-beam treatment (IBT), which is normally used in semiconductor technology for surface milling. It has been shown that such a treatment can lead to the conductivity type conversion in p-type HgCdTe with conversion depths exceeding ion paths by many orders of magnitude, resulting in n-type layers of excellent quality [1–6]. Conductivity type conversion in HgCdTe by IBT has been reported mostly in alloys with composition x ≤ 0.24 (bandgap Eg ≤ 0.15 at 77 K), which are the materials of choice for long-wavelength IR detectors. Most recently, the conversion has been observed in alloys with composition ∗

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x ∼0.31 subjected to ion-beam [6] and plasma treatment [7], but the effect of the composition on conversion depth has not been studied as yet. In this paper, we report on the conductivity type conversion by IBT in HgCdTe epitaxial layers with x up to 0.39 (Eg ≤ 0.41 eV at 300 K, which corresponds to the mid-IR part of the spectra). Also, conversion caused by IBT is studied in quaternary Hg1−y−z Zny Cdz Te alloys with 0.15 ≤ Eg ≤ 0.42 at 300 K. These compounds are believed to be an alternative for HgCdTe, as the presence of Zn in the alloy stabilise the weak Hg–Te chemical bond and should lead to the improvement of material quality. Up to now, only ternary Hg1−y Zny Te compounds have been subjected to IBT [8,9]. 2. Experimental The epitaxial layers were grown on CdTe substrates by liquid phase epitaxy technique from Te-rich solutions. After the growth, the layers were subjected to annealing in saturated mercury vapour. A p-type material with Na − Nd concentration (defined as a hole concentration at 77 K) in the range (4.6/380) × 1015 cm−3 was obtained as a result of annealing at temperatures from 300 to 370 ◦ C. IBT was performed in a vacuum chamber with a neutralized beam of

K.D. Mynbaev, V.I. Ivanov-Omskii / Journal of Alloys and Compounds 371 (2004) 153–156

Ar ions with energy of 60–800 eV and ion current density of 0.05–0.2 mA/cm2 . No intentional cooling was used during the treatment. To determine the conversion depth and to study the electrical properties of the converted layers, the Hall coefficient and DC conductivity were measured on the treated samples with 0.7 ␮m step chemical etching. The measurements were performed at 77 K for HgCdTe layers with x < 0.4 and HgZnCdTe layers and at 300 K for HgCdTe samples with x > 0.5. Prior to the measurements, a 1 ␮m-thick ion-damaged layer [2] was chemically etched from the surface of the samples.

16 14 12

h, µm

154

10 8 6 0.28

0.30

0.32

0.34

0.36

0.38

0.40

x

3. Results and discussion Table 1 presents the results of IBT-induced modification in p-type HgCdTe epitaxial layers with 0.21 ≤ x ≤ 0.55 treated with Ar ions with the energy of 400 eV at 0.1 mA/cm2 current density for 20 min. As seen from this Table, the conversion took place in all the samples with x ≤ 0.39. Low electron concentration n and high mobility of electrons µn point to the high quality of the n-layer obtained. Electrical properties of the converted layers did not change with the energy of the ions and current density. Conversion in HgCdTe layers with x > 0.5 did not occur under any condition. Since in these experiments, the conversion was achieved in samples with different x, the dependence of the conversion depth h on x could be established. The samples used had different initial Na − Nd values, yet previously we have established [3] that h is inversely related to (Na − Nd )1/2 , so when plotting h(x) dependence, h values were reduced to initial concentration (Na − Nd )0 = 1.0 × 1016 cm−3 . Fig. 1 demonstrates that h decreases with x. The plot can be fitted with a following equation:
x  h ≈ 6.32 + 2.48 × 104 exp − , 0.036 where h is the conversion depth in micrometers.

Fig. 1. The dependence of the conversion depth on the alloy composition x for HgCdTe layers subjected to IBT.

The observed relation between h and x is in a good agreement with the diffusion model [3,10–12] of conductivity type conversion in HgCdTe under IBT. This model states that the conversion is due to the diffusion of interstitial mercury atoms HgI , which are freed at the surface of the crystal sputtered by ion treatment. These atoms are believed to diffuse into the crystal and annihilate the mercury vacancies, which are acceptors and were defining initial p-type of conductivity of the material. According to this model, h2 /t ∼ Cs D/(Na − Nd ), where D is the diffusion coefficient, Cs is the surface concentration of HgI , and t is the treatment time. Obviously, for such a strong dependence of h on x as the one presented in Fig. 1, Cs should decrease drastically with increasing x. This, however, can be easily explained. First of all, it is known that the rate of ion-beam sputtering of HgCdTe (and therefore, the rate of the generation of free atoms on the surface) decreases with increasing x [13]. Naturally, with x increasing, a fraction of Hg atoms in the total amount of atoms released from the surface also decreases. Also, with increasing x, and therefore, Eg , the built-in electric field

Table 1 Electrical properties of Hg1−x Cdx Te layers subjected to IBT Sample

Eg (eV), 300 K

x

Before IBT Na − N d

C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12

0.165 0.179 0.193 0.264 0.291 0.304 0.318 0.412 0.412 0.565 0.594 0.637

0.21 0.22 0.23 0.28 0.30 0.31 0.32 0.39 0.39 0.50 0.52 0.55

1.6 1.1 2.2 8.0 1.1 7.3 3.2 1.8 2.2 2.1 2.4 2.3

× × × × × × × × × × × ×

(cm−3 )

1016 1017 1016 1015 1016 1015 1016 1016 1016 1016 1016 1016

After IBT µp

(cm2 /V s)

530 240 470 360 420 460 420 250 260 75 120 260

n (cm−3 ) 5.8 1.0 7.8 1.0 1.4 6.3 2.7 1.1 8.8 No No No

× 1014 × 1016 × 1014 × 1015 × 1015 × 1014 × 1015 × 1015 × 1014 conversion conversion conversion

µn (cm2 /V s) 1.9 1.2 5.6 3.7 6.2 2.1 4.9 1.9 2.0

× × × × × × × × ×

105 104 104 104 104 104 103 104 104

K.D. Mynbaev, V.I. Ivanov-Omskii / Journal of Alloys and Compounds 371 (2004) 153–156

16

10

1 -3

2

n77, cm

effect is enhanced. This field appears near the boundary of the converted layer due to a graded density of charged centres and is expected to decrease the efficient Cs value [11]. Electrical properties of HgZnCdTe layers after IBT were similar to those of HgCdTe samples. Eg value in these samples, as determined from optical transmission studies at 300 K, varied from 0.146 to 0.421 eV. Conversion was observed in all the samples, and the electron concentration in the layers after IBT varied from 5.8 × 1014 up to 3.5 × 1015 cm−3 . The highest electron mobility value in HgZnCdTe layer was observed for the sample with Eg = 0.185 eV and equalled 1.3×105 cm2 /V s. Thus, in this work we have observed conductivity type conversion in HgCdTe layers with x ≤ 0.39 (Eg ≤ 0.41 eV) and HgZnCdTe with Eg ≤ 0.42 eV, treated with a neutralized Ar ion beam. Let us note that in our early works [2,3] we have not achieved the conversion in HgCdTe epitaxial layers with x > 0.24. However, in those experiments a charged ion beam was used with samples having been fixed to an insulating sample holder. Therefore, the conversion in the HgCdTe layers with x > 0.24 must be dependent on whether a charged or a neutralized ion beam is used. After switching off the ion beam neutralisation system in our experimental set-up, we observed no conversion in HgCdTe layers with x ≥ 0.28, which confirmed the suggestion. The reason for this effect may be quite simple. When a charged ion beam is used, a charged layer appears on the surface of the treated sample. In narrow band-gap semiconductor, which is the case for HgCdTe with x ∼ 0.2, this charge is compensated by intrinsic carriers, whose concentration in these materials at the treatment temperature (T ∼ 330 ◦ C) is rather high. In wider band-gap material, with bigger Eg , intrinsic carrier concentration is much less and they are not able to compensate the charge created by ions. This charge then accumulates on the surface and repels the ion beam. Moreover, the presence of this charge on the surface superimposes on the above mentioned built-in electric field effect and decreases effective Cs value to the point where no conversion occurs. With x > 0.5, the decrease of Hg content in the layers, as well as the electric field effect, decrease Cs so drastically that even the neutralized ion beam does not produce the conversion. A similar effect was observed for HgZnCdTe epitaxial layers, where the charged ion beam did not cause the conversion in samples with Eg ≥ 0.344 eV, while a neutralized beam created n-type layers of several micrometers in depth. Reverting to Table 1, we should like to point out that its data confirm our earlier suggestion [3] that the electron concentration n in the converted HgCdTe layers is related to the acceptor concentration Na − Nd as measured in the layers before the IBT. In fact, considering this relation as presented in Fig. 2, we can now show a clear dependence of n on Na − Nd . It is seen in this figure that for both groups of the samples studied (with x ∼ 0.2 and x ≥ 0.3), n increases linearly with Na − Nd . For x ∼ 0.2 n/(Na − Nd ) ∼ 1/10, while for x ≥ 0.3 the slope of the line drops. Thus, the de-

155

15

10

14

10

15

10

16

17

10

10 -3

Na-Nd, cm

Fig. 2. The dependence of electron concentration in the converted layers on the acceptor concentration in the annealed layers before IBT: 1 for x ∼ 0.2, 2 for x ≥ 0.28.

pendence confirms the suggestion that the electron concentration in the converted layers is determined not only by the background impurities as was proposed in [1], but also by intrinsic donor defects. The concentration of these defects, as follows from Fig. 2, is bound to increase with Na − Nd . That means that during the thermal annealing, the generation of acceptor defects, which are mercury vacancies, is accompanied by the generation of some amount of donor intrinsic defects, i.e., there is a self-compensation effect. Note, that self-compensation of intrinsic acceptors in HgCdTe was suggested earlier on the basis of indirect evidence, such as the carrier transport [14,15] and excess carrier lifetime measurements [16]. To satisfy the law of mass action, it was suggested that these donor defects might be Te antisites on metal vacancies [15]. The decrease in the slope of the n(Na − Nd ) dependence for the layers with x ≥ 0.3 can be explained by the fact that the effectiveness of self-compensation depends on the energy gap, decreasing with the latter increasing. Therefore, the results obtained in this work give direct evidence to the self-compensation effect and also prove that IBT can be used as a unique tool for defect study in HgCdTe. Also, the results presented in Fig. 2 shows that there is the possibility of controlling the electron concentration in the converted layers, and therefore, to use this method for fabricating p–n junctions with desired concentrations in both p- and n-regions—something, which the method had seemed to lack when it was first introduced. This makes low-energy ion-beam treatment very promising for becoming a state-of-the-art method for fabricating IR devices based on HgCdTe. 4. Conclusion In conclusion, we have studied modification of p-Hg1−x Cdx Te and p-Hg1−y−z Zny Cdz Te alloys with various com-

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K.D. Mynbaev, V.I. Ivanov-Omskii / Journal of Alloys and Compounds 371 (2004) 153–156

positions by low-energy ion-beam treatment. Conductivity type conversion was observed as a result of the treatment of the HgCdTe alloys with x < 0.4. The conversion depth decreased with x increasing. It was shown that for conductivity type conversion in Hg1−x Cdx Te with 0.28 ≤ x ≤ 0.39 it was essential to use a neutralized ion beam. The electron concentration in the converted layer increased with the acceptor concentration in the material before the treatment, which confirmed the effect of self-compensation of intrinsic acceptor defects by intrinsic donor defects during the annealing prior to ion treatment. This demonstrates the possibility of controlling the carrier concentration in p–n junctions fabricated by ion-beam treatment in vacancy-doped Hg1−x Cdx Te.

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