With excellent electrical and optical properties, III–V compound semiconductors have become fundamental materials for the photoelectronic and microelectronic industries, among which gallium antimonide (GaSb) is of particular importance. GaSb-based devices such as laser diodes, photovoltaic cells, microwave devices, and infrared (IR) detectors are promising for applications in mid-infrared regions.^[1–5] GaSb has also become an excellent substrate for epitaxial growth of various ternary and quaternary III–V compounds due to their well-matched lattice parameters.^[6–9]

Irrespective to crystal growth methods, undoped GaSb is p-type with an intrinsic acceptor defect concentration up to 10¹⁷ cm⁻³.^[10,11] This high concentration of native defects in GaSb substrates may lead to an enhancement of IR absorption and electrical compensation which has a negative effect on fabricating high performance IR detectors. The p-type conductivity in undoped GaSb is usually believed to arise from gallium vacancies (V_Ga) and gallium in antimony site (Ga_Sb). These acceptors are sometimes seen as V_GaGa_Sb complexes with a doubly ionizable nature.^[12,13] Other researchers such as Ling et al.^[14] and Slotte et al.^[15] have ruled out the V_Ga-related origin hypothesis using positron annihilation spectroscopy. Unlike GaAs and InP,^[16,17] GaSb has a narrow bandgap of 0.73 eV with no deep donors nor deep acceptors, thus it may not be possible to make GaSb semi-insulating for manufacturing high-frequency devices. GaSb single crystal with various resistivities can be obtained by controlling the compensation degree via doping. In addition, GaSb single crystal with high compensation, which has a relatively high resistivity, exhibits better transmittance in the infrared region.^[18,19] However, GaSb single crystals grown by liquid encapsulated Czochralski (LEC) method usually can only achieve a maximum carrier concentration around 8×10¹⁷ cm⁻³.

Researches on the compensating mechanism and doping concentration have been developed for decades and still need to be continued. In this paper, to comprehend the mechanism of defect compensation and the effects of impurity on the electrical and optical properties, undoped and Te-doped GaSb single crystal samples were prepared by the LEC method. The concentrations of Te dopant in the samples were measured by glow discharge mass spectrometry (GDMS). The electrical and optical properties of the samples were analyzed by Hall effect measurements and photoluminescence (PL) spectroscopy. The result shows evidence that the doping of Te impurity may promote the ionization of the native acceptors and slow down the compensation process.

Undoped and Te-doped GaSb single-crystal ingots were grown by the LEC method using high-purity (99.9999%) Ga and Sb metals as the raw materials. Various doses of high-purity Te elementary substance were used as dopant to achieve different doping levels. After growth, 2-inch [100] oriented substrates were then cut from these ingots and applied with chemical mechanical polishing on both sides. The substrates were later washed in a series of organic and inorganic solutions to be epi-ready. The samples used in this work were all sliced into 1 cm×1 cm squares before other measurements.

The Hall effect measurements were carried out at 300 K using the Van der Pauw method to test and analyze the electrical properties of different GaSb samples. A four-point probe was placed at the four corners of the square samples. By soldering indium dots to the surfaces of the samples, ohmic contacts could be achieved.

To determine the concentrations of Te in the doped samples, GDMS measurements were taken using a VG9000 system which provides a ppb/ppt (weight) level detection limit.

For PL spectra, a Bruker Vertex 80 V Fourier infrared spectrometer (FTIR) with a 647 nm Kr⁺ laser excitation source was used. And a HgCdTe photodetector with a resolution of 0.5 meV was employed to detect the signal. The PL measurements were all taken at 10 K.

For Te-doped GaSb samples at different doping levels, the Te concentrations measured by GDMS as well as the electrical properties gained from the Hall effect measurements are listed in Table 1. The Te-doped samples show n-type conductivity with a net electron concentration varying from as low as 3.09×10¹⁶ cm⁻³ to as high as 7.50×10¹⁷ cm⁻³.

Table 1.

Table 1.

Table 1. Te concentrations measured by GDMS and electrical properties of Te-doped samples. . Sample Sampling position Te concentration/cm−3 Type Carrier Ionized acceptor Mobility of ingot concentration/cm−3 concentration/cm−3 / G01H head 2.41×1017 n 3.09×1016 2.10×1017 3.04×103 G02H head 5.04×1017 n 1.24×1017 3.80×1017 2.44×103 G03H head 7.42×1017 n 3.35×1017 4.07×1017 3.15×103 G04H head 7.16×1017 n 4.14×1017 3.02×1017 3.24×103 G05T tail 1.93×1018 n 7.50×1017 1.18×1018 2.50×103

Table 1. Te concentrations measured by GDMS and electrical properties of Te-doped samples. .

Basically, the carrier concentration increases with the Te concentration. Te is a shallow donor with an impurity energy level above the conduction band minimum. We assume that the donor impurities are fully ionized at 300 K, therefore the ionized donor concentration roughly equals the donor concentration, i.e., , and the ionized acceptor concentration , depending on the ionization degree of the doubly ionizable native acceptor. After linear fitting of the data, in relatively lightly doped samples, which has a Te concentration lower than 10¹⁸ cm⁻³, the net carrier concentration can be estimated as

Based on the dosages of Te elementary substance added to the raw materials in the crystal growth process, the approximate Te concentrations in the Te-doped samples can also be calculated using a modified version of Scheil equation^[20]

Table 2 shows the calculated Te concentrations of more undoped and Te-doped samples using Eq. (2) and their net carrier concentrations measured at 300 K. For convenience of calculation, the Te-doped samples are taken from the head of the ingots. The relationship between the net carrier concentration and the calculated Te concentration can be approximated by

Fig. 1.

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	Fig. 1. Carrier concentration as a function of Te concentration. Triangles: data gained from the GDMS and Hall effect measurements. Squares: data gained from Te concentration calculation and Hall effect measurements, straight line: data linear fitted by Eq. (1), dashed line: data linear fitted by Eq. (3), dotted line: best fit curve for data gained from the GDMS and Hall effect measurements.

Table 2.

Table 2.

Table 2. Calculated Te concentrations and electrical properties of undoped and Te-doped GaSb samples. . Sample Sampling position Te concentration/cm−3 Type Carrier Ionized acceptor Mobility of ingot concentration/cm−3 concentration/cm−3 / G06H head 0 p 1.58×1017 1.58×1017 6.13×102 G06T tail 0 p 1.58×1017 1.58×1017 6.14×102 G07H head 2.62×1017 n 1.81×1016 2.44×1017 1.76×103 G08H head 2.71×1017 n 3.09×1016 2.40×1017 3.04×103 G09H head 3.80×1017 n 1.24×1017 2.56×1017 2.45×103 G10H head 3.84×1017 n 1.28×1017 2.56×1017 3.23×103 G11H head 6.87×1017 n 3.35×1017 3.52×1017 3.15×103 G12H head 7.11×1017 n 3.49×1017 3.62×1017 2.89×103

Table 2. Calculated Te concentrations and electrical properties of undoped and Te-doped GaSb samples. .

Because of the volatility of Sb during the process of crystal growth, a Ga-rich condition is usually obtained and Sb vacancies (V_Sb) are easy to form under this condition. The Ga_Sb antisite is found to have a fairly low formation energy of 1.37 eV,^[24] thus, the V_GaGa_Sb acceptor complexes can originate from Sb vacancies through the nearest-neighbor diffusion mechanism^[25] as

Meanwhile, the carrier mobility listed in Tables 1 and 2 shows a growth and decline as the Te concentration increases. For samples with a net carrier concentration lower than 1.28×10¹⁷ cm⁻³, the mobility increases with the Te concentration. This may be caused by the electron screening effect as the electrons fill up the higher lying conduction band valley.^[26] When the Te concentration keeps increasing, more native acceptors and Te dopants are ionized, thus the ionized-impurity scattering becomes the dominant scattering mechanism and the electron–electron scattering also comes into effect, resulting in the decrease of mobility.

Figure 2 shows the variation of the ionized acceptor concentration whose overall line shape is near hyperbolic. The slope of the asymptotic line at low Te concentration is 0.28 corresponding to Eq. (3). The growth of ionized acceptor concentration accelerates at a higher doping level. For the asymptotic line at high Te concentration, its slope should be close to 1 if the maximum carrier concentration is cm⁻³.

Fig. 2.

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	Fig. 2. The variation of ionized acceptor concentration. Squares: calculated concentration of ionized acceptors at different Te concentration. Dotted line: asymptotic lines for the fitted hyperbolic curve.

This result can be further verified by our PL spectra demonstrated in Fig. 3. For the undoped GaSb sample, we see an intense peak at 779 meV commonly known as band A corresponds to the doubly ionizable native acceptor defect V_GaGa_Sb at its neutral state.^[27–29] A weaker peak, known as band G, at 756 meV is caused by another acceptor associated with Ga_Sb.^[28,30] This peak annihilates in the lightly doped samples suggesting that the Ga_Sb defects are suppressed by Te compensation. Other research has shown that band G could also be eliminated when the GaSb samples were annealed in Sb ambient.^[31] And band D which shows a peak at 797 meV can be assigned to the reported excitons bound to the neutral acceptor.^[32]

Fig. 3.

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	Fig. 3. PL spectra of undoped and Te-doped GaSb samples measured at 10 K. The undoped GaSb sample is p-type. The net carrier concentrations of the Te-doped GaSb samples listed on the left are measured at room temperature. The same emission peaks of different PL curves are linked by dotted lines.

For lightly doped GaSb samples, a broad band containing band C and band T appears. This combination then becomes the main peak with the increase of Te doping level. It has been suggested that band C corresponds to the singly ionized native acceptor (V_GaGa_Sb)⁻, and band T arises from a Te_Sb donor complexing with a doubly ionized (V_GaGa_Sb)²⁻ center.^[28] The specific process can be described as follows:

For the heavily doped sample, the intensity of the broad peak sees a significant enhancement, which means the amount of all the ionized acceptors is way more than that in the lightly doped samples. This change corresponds to the decrease of the slope in Fig. 1 and confirms that a high concentration of Te promotes the formation of more V_GaGa_Sb acceptor defects.

In addition, both band C and band T show a blue shift as the doping concentration increases. This phenomenon can be explained by the Moss–Burstein effect.^[33] GaSb is easy to become degenerate at relatively low electron densities, as the doping level get higher, the Fermi level rises into the conduction band, which increases the optical band gap.

Comparing the doped samples to the undoped sample, the intensities of band A and band D decrease rapidly, and the two peaks merge into a new one we call band K at the position of 787 meV. Rather than annihilating immediately, this newly formed peak weakens gradually which proves that although Te impurity at a concentration much higher than that of the initial native acceptors has been doped into the GaSb crystal, a certain number of uncompensated native acceptors still exist.

We prepared undoped and Te-doped GaSb samples by the LEC methods. The net carrier concentration versus Te concentration (fitted lines) shows that the defect compensation process lags behind the impurity doping process and the net carrier concentration tends to saturate, indicating that the introduction of Te dopants may promote the formation and ionization process of the doubly ionizable V_GaGa_Sb acceptor complexes. The results of the PL spectra further corroborate this hypothesis. At relatively low doping levels, when the Te concentration increases, more V_GaGa_Sb acceptor centers are ionized and new acceptor complexes of (V_GaGa_SbTe_Sb)⁻ form. As the doping level keeps growing, more V_GaGa_Sb acceptor complexes are generated, making the compensating progress slower.