INTRODUCTION

Photosensitive cadmium selenide layers have recently attracted considerable attention in connection with the advent of new classes of solid-state optoelectronic elements, including position-sensitive semiconductor photodetectors (PSSPs) [1–3].

It appears promising to grow CdSe layers to be used as basic photosensitive components of PSSPs. Thermal evaporation in a quasi-closed system (QCS) makes it possible to grow oriented CdSe layers on mica [4, 5]. Muscovite mica was chosen as a substrate material because it is well studied in relation to CdSe layer growth [5] and offers the possibility of producing layers on a large area of a mica cleavage surface for practical application in engineering [5]. The layer growth temperature was chosen so as to minimize the evaporation temperature (640–660°C): high evaporation temperatures lead to contamination of growing layers. In addition, we aimed at growing CdSe layers having high photosensitivity and low carrier concentration, suitable for use as PSSP materials, and exhibiting excitonic luminescence.

The objectives of this work were to grow CdSe layers on muscovite mica in a QCS, determine their electrical transport characteristics, and investigate their excitonic cathodoluminescence spectra.

EXPERIMENTAL

Cadmium selenide was synthesized by a two-zone method in quartz ampules sealed off under vacuum (p = 10–4 Pa). As starting materials, we used Kd-000 cadmium and OSCh 22-4 selenium, which were further purified by vacuum distillation.

The composition of the as-prepared cadmium selenide crystals was brought to the one corresponding to the minimum pressure pmin by sublimation under dynamic vacuum. The condensation zone of selenium and cadmium—highly volatile substances—was in the tail fraction, with removal of excess cadmium and selenium and other highly volatile impurities. In this process, low-volatile impurities, such as Fe, Co, Ni, and others (M), were also removed from CdSe. MSe compounds are nonvolatile. On heating, they do not vaporize but decompose to give Se(v) and M(s). Low-volatile impurities accumulate in the residue.

CdSe layers were grown by thermal evaporation in a QCS, using a UVN-MR-2 standard vacuum apparatus (p ≤ 10–4 Pa). The evaporator temperature was Tev = 933 or 913 K. The layers were grown on freshly cleaved (001)-oriented muscovite mica surfaces at temperatures from 703 to 853 K. We used mica (muscovite) stable at temperatures of up to 863 K (according to its data sheet). Preliminary experiments showed that the muscovite samples did not decompose during heating in vacuum at a temperature of 855 ± 0.5 K for 25 min (layer growth time). During layer growth, the evaporator temperature was 913 or 933 K. The thermal conditions of layer growth in the QCS were monitored, stabilized, and controlled using a specially designed experimental setup [6]. The temperature was maintained with an accuracy of ±0.5 K. The optimal layer growth time was 20–25 min. It is worth noting that the growth of CdSe layers was accompanied by the removal of both highly volatile and low-volatile impurities.

Micrographs of the surface of the layers were taken on an MII-4 microscope [7].

The thickness of the layers was determined using a Linnik microinterferometer.

The phase composition and surface condition of the layers were determined by X-ray diffraction. Intensity data were collected on a DRON-4 diffractometer (CuKα radiation). X-ray diffraction patterns were analyzed using WinXPOW software. To avoid recording diffraction peaks of the substrate, prior to data acquisition the CdSe layers were separated from the mica, without damaging them. To obtain data on the phase composition of the grown CdSe layers, they were ground into powder before X-ray diffraction characterization.

The resistivity of the CdSe layers and the carrier concentration and mobility in them were determined by a modified four probe van der Pauw method [8]. The carrier concentration, mobility, and type in the CdSe layers were determined by Hall effect measurements. The Hall voltage across the samples was measured at a temperature of 300 K and magnetic field induction of 0.96 T. The conductivity type was inferred from the sign of the Hall voltage.

At a working current of 1 μA, uncertainties in our resistivity and Hall coefficient measurements were no greater than 10%. The current passing through the semiconductor layer was measured with an accuracy of ±0.1 μA. The voltage between the central probes of the measuring head was measured with an accuracy of ±1 μV.

In our measurements of the photocurrent to dark current ratio, a photometered 400-W incandescent tungsten lamp was used as a light source.

Cathodoluminescence (CL) spectra were measured at 78 K. The luminescence was excited by a pulsed 40-keV electron beam. The CL spectra were detected using a DFS-13 monochromator.

RESULTS AND DISCUSSION

CdSe layer growth. All of the grown cadmium selenide layers had the wurtzite structure (stable polymorph of CdSe). The surface area of the CdSe layers was ~3 cm2. The thickness of the textured layers ranged from 6 to 50 μm.

To analyze our results, we used the parameter γ = (Tev – Ts)/Tev (where Tev is the evaporation temperature and Ts is the substrate temperature) (Fig. 1).

Fig. 1.
figure 1

Regions of the growth of (1) CdSe layers with the only texture (0001), (2) layers with two textures, and (3) polycrystalline layers on mica. The numbers at the data points specify the corresponding coefficient γ.

Full size image

On the surface of the cadmium selenide layers grown at 743 K (evaporation temperature of 913 K), we observed hexagonal grains 180 μm in linear size (Fig. 2). These layers were textured: their X-ray diffraction patterns showed only the \(10\bar {1}3\) and \(10\bar {1}5\) lines.

Fig. 2.
figure 2

Micrograph of the surface of the CdSe layers grown on mica at a substrate temperature Ts = 763 K and evaporator temperature Tev = 913 K.

Full size image

Further raising the substrate temperature, in the range 793–833 K, at an evaporation temperature of 933 K led to an increase in the size of the hexagonal grains to 400 μm. As the substrate temperature was raised to 853 K (Tev = 933 K) and, accordingly, γ was reduced to 0.08, the size of the hexagons reached 600 μm. The layers had only (0001) texture, and their X-ray diffraction patterns showed only one line: 0002 (Fig. 3).

Fig. 3.
figure 3

X-ray diffraction pattern of the CdSe layers grown on mica at Ts = 853 K and Tev = 933 K.

Full size image

The layers grown at Tev = 933 K and Ts = 833 K (γ = 0.10) were textured. The dominant texture was \(\left( {10\bar {1}3} \right)\), with a slight amount of a \(\left( {10\bar {1}5} \right)\) texture.

The layers grown at an evaporation temperature of 913 K and γ = 0.16 or 0.18 had a combination of two textures. At γ = 0.21–0.23, we obtained polycrystalline cadmium selenide layers.

Electrical transport properties of the CdSe layers and their dependence on the substrate and evaporator temperatures. All of the CdSe layers grown by us were n-type. The dark resistivity ρd of the CdSe/mica layers grown at an evaporation temperature of 933 K was found to increase sharply with increasing substrate temperature (Fig. 4, curve 2).

Fig. 4.
figure 4

Dark resistivity ρd (at 300 K) as a function of substrate temperature for the CdSe layers grown at evaporation temperatures of (1) 913 and (2) 933 K.

Full size image

The CdSe layers had high sensitivity to unfiltered light (Fig. 5, curve 2). At 300 K and an illuminance of 200 lx, the photocurrent to dark current ratio of the layers grown at a substrate temperature of 853 K and evaporation temperature of 933 K was 6.2 × 104.

Fig. 5.
figure 5

Photocurrent to dark current ratio as a function of substrate temperature for the CdSe layers grown at evaporation temperatures of (1) 913 and (2) 933 K.

Full size image

Figure 6 shows carrier (electron) concentration (curves 1) and carrier mobility (curves 2) as functions of substrate temperature for the CdSe layers. In the case of high-temperature growth conditions (Tev = 933 K and Ts = 853 K), the carrier concentration was as low as 9 × 1011 cm–3. This extremely low carrier concentration attests to high purity of the CdSe layers, due to the removal of unintentional impurities during the layer growth process, and indicates that their composition approaches the stoichiometric one.

Fig. 6.
figure 6

Electron (1) concentration and (2) mobility as functions of substrate temperature for the CdSe layers grown at evaporation temperatures of 913 (Ts = 700–770 K) and 933 K (Ts = 790–853 K).

Full size image

Excitonic luminescence spectra of the CdSe layers. Excitonic luminescence was observed at photon energies from 1.810 to 1.797 eV. The 78-K CL spectrum of the CdSe layers grown at Tev = 933 K and Ts = 853 K (γ = 0.08) contained only one line (in the case of measurements in the wavelength range from 670 to 1250 nm), due to free A-excitons (Fig. 7a). The presence of a single CL line, corresponding to free A-exciton annihilation, points to high purity of the CdSe layers and indicates that the composition of the layers approaches the stoichiometric one.

Fig. 7.
figure 7

78-K excitonic luminescence spectra of the CdSe layers with only (0001) texture (a) and the layers having two textures and grown at an evaporation temperature of 933 K and substrate temperatures of 833 (b) and 813 K (c).

Full size image

The XA line (EA = 1.810 eV) is related to the luminescence due to phonon-free annihilation of free A‑excitons in their ground state, with a principal quantum number n = 1 [9]. The characteristic full width at half maximum of the XA line was 0.012 eV, and the calculated binding energy Ex of the free A-excitons in their ground state was 0.016 eV, in agreement with previously reported data [9]. Cadmium selenide is a direct band gap semiconductor. Its band gap Eg can then be evaluated using the relation EA = Eg – Ex [9] (EA = 1.810 eV). At 78 K, the cadmium selenide layers have Eg = 1.826 eV.

The spectra of the other textured cadmium selenide layers, grown at an evaporation temperature of 933 K, also showed evidence for excitonic luminescence. In particular, we observed the free A-exciton line at 1.810 eV, with a relatively low intensity in the case of the textured layers grown at a substrate temperature of 833 K (Tev = 933 K, γ = 0.10) (Fig. 7b). The spectrum of the textured layers grown at a substrate temperature of 813 K (Tev = 933 K, γ = 0.13) has no free A-exciton line (Fig. 7c). At the same time, the spectra in Figs. 7b and 7c contain a strong band at 1.797 eV. The peak position of the 1.797-eV band corresponds to the first LO phonon replica of free A-excitons: XA-1LO [9]. The 1.797-eV band is broader than the free-exciton line (Fig. 7a). The point is that the XA-1LO line (1.797 eV) overlaps with a line of excitons bound to donors, namely, to selenium vacancies (VSe) and interstitial cadmium atoms (Cdi), which are intrinsic defects in cadmium selenide. In luminescence spectra, the peak position of the line of donor-bound excitons is 1.7995 eV [10], and this line is probably a component of the main band XA-1LO (1.797 eV). Thus, the large full width at half maximum of the 1.797-eV band (Fig. 7) suggests the existence of a bound-exciton line and the presence of donor centers (VSe and Cdi) in cadmium selenide crystals located in the textured layers grown at an evaporation temperature of 933 K and substrate temperatures of 813 and 833 K.

The cadmium selenide layers grown at an evaporation temperature of 913 K exhibit no excitonic luminescence. The spectra of these layers contain bands in the range 1.67–1.72 eV, typical of spectra of undoped polycrystalline powder cadmium selenide.

CONCLUSIONS

Textured CdSe layers have been grown on muscovite mica by thermal evaporation in a QCS.

We have demonstrated that the 78-K CL spectra of the cadmium selenide layers grown on mica at an evaporation temperature of 933 K and a substrate temperature of 853 K have a single line, corresponding to radiative annihilation of free A-excitons. The spectra of the layers grown at 833 K show lines of free A-excitons and their first phonon replica.

The CdSe layers grown at a substrate temperature of 853 K (and an evaporation temperature of 933 K) have been shown to have high photosensitivity (photocurrent to dark current ratio of 6.2 × 104) and an extremely low carrier concentration (9 × 1011 cm–3), which is of interest for producing PSSP materials based on such layers.