CN104532172A - Heat treatment method for eliminating tellurium-rich precipitate-phase defect in tellurium-zinc-cadmium material through two-step process - Google Patents

Abstract

The invention discloses a two-step heat treatment method for eliminating tellurium-rich precipitate phase defects in cadmium zinc telluride materials. In the method, the cadmium zinc telluride material is placed in a tellurium-rich state for heat treatment, so that excess tellurium in the tellurium-rich precipitate phase defects The atoms are discharged from the sample, and then the sample is treated with cadmium-rich heat treatment, so that the Cd atoms enter the liquid tellurium-rich precipitate phase, and the size of the defect in the tellurium-rich precipitate phase is reduced by using the supersaturated epitaxy process of the liquid tellurium-rich precipitate phase . The conventional heat treatment method reduces the size of the tellurium-rich precipitated phase defect, and at the same time generates a large number of misfit dislocations in the surrounding material of the precipitated phase defect. In contrast, in the method of the present invention, since the content of tellurium atoms in the tellurium-rich precipitation phase is controlled during the tellurium-rich heat treatment process, this process will not cause stress and misfit dislocations to surrounding materials. The two-step heat treatment technology can effectively improve the quality of the material and meet the needs of the application of CdZnTe materials as optoelectronic devices or substrate materials.

Description

Two-step heat treatment method for eliminating defects in tellurium-rich precipitated phases in CdZnTe materials

technical field

The invention relates to a manufacturing process technology of a cadmium zinc telluride material, in particular to a two-step heat treatment method for eliminating defects of tellurium-rich precipitation phases in the cadmium zinc telluride material.

Background technique

CdZnTe material is an important semiconductor material. It can be used as the substrate of HgCdTe epitaxial material to prepare and produce high-performance infrared focal plane detectors, and can also be directly used in the preparation and production of high-energy sensors that sense gamma rays. Ray detector. Infrared focal plane detectors and γ-ray detectors are widely used in aerospace remote sensing, medical equipment, security inspection and military equipment.

Compared with the widely used Si material and GaAs material, the defect formation energy of CdZnTe material is very low, and the thermal conductivity is very low. Most of the grown CdZnTe ingots are multi-grain materials, and more or less Contains tellurium-rich or cadmium-rich precipitate phase defects, the defect density is 10 ³ ~ 10 ⁵ cm ^-3 , and the size is generally between 5 ~ 30 μm. Monocrystalline wafers are cut from large grains, and defects in the material need to be adjusted through a heat treatment process. The research results of Vydyanath ^[1] , Sen ^[2] and Belas ^[3] all show that the heat treatment process using the cadmium-rich state (the crystal in equilibrium with it is in a cadmium-rich stoichiometric state) can effectively reduce the tellurium-rich state. The size of the precipitated phase defects, but in our research on the elimination of the tellurium-rich precipitated phase defects in CdZnTe materials, we found that the cadmium-rich heat treatment can reduce the tellurium-rich precipitated phase defects in the CdZnTe material at the same time, around the precipitated phase defects There will be a serious dislocation propagation effect in the material (see Figure 1). It can be seen from the figure that after heat treatment, the defects in the tellurium-rich precipitate phase are significantly reduced, but a large number of defects appear around the precipitate phase defects. Dislocations, whose range is roughly 5 to 10 times that of precipitated phase defects, that is, while heat treatment reduces the size of precipitated phase defects, the appearance of dislocation proliferation regions also brings new damage to the material quality, which will lead to material The decrease of the minority carrier lifetime eventually leads to the degradation of the detector performance.

references:

[1] H.R.Vydyanath, J.Ellsworth, J.J.Kennedy, et al.Recipe to minimize Teprecipitation in CdTe and (Cd, Zn)Te crystals[J].Journal of Vacuum Science&Technology B, 1992, 10:1476-1484

[2]S.Sen, C.S.Liang, D.R.Rhiger, et al.Reduction of CdZnTe substrate defects and relation to epitaxial HgCdTe quality[J].J.Electron.Mater.1996,25:1188-1195

[3]E.Belas, M.Bugár, R.Grill, et al.Reduction of inclusions in(CdZn)Te andCdTe:In single crystals by post-growth annealing[J].J.Electron.Mater.2008, 37: 1212-1218

[4]Everson W J, Ard C K, Sepich J L, et al. Etch pit characterization of CdTe and CdZnTe substrates for use in Mercury Cadmium Telluride epitaxy[J].Journal of Electronic Materials,1995,24(5):505-510

[5] JHGREENBERG, VNGUSKOV, M.FIEDERLE, et.al., Experimental Study of Non-Stoichiometry in Cd _1-x Zn _x Te _1-δ , Journal of ELECTRONIC MATERIALS, 33(6), 2004: 719-723

Contents of the invention

Aiming at the problem that cadmium-rich heat treatment produces dislocations on surrounding materials of tellurium-rich precipitate defects, the present invention proposes a two-step heat treatment method for tellurium-rich materials, which solves the problem of reducing the size of tellurium-rich materials while reducing the size of precipitate phase defects. The problem of dislocation value increase occurs, so that the quality of tellurium-rich materials can be improved more effectively. This method first heat-treats the Teurium-rich CdZnTe material in a tellurium-rich state to remove the excess Te atoms in the tellurium-rich precipitate phase from the material, and then heat-treats the material in a cadmium-rich state to make the defects in the tellurium-rich phase Under the action of Cd partial pressure, it enters a supersaturated state, and then epitaxy occurs on the inner wall of the material (outside the defect region), so as to achieve the purpose of reducing the defect size of the precipitated phase without generating dislocation increment. The invention provides the following technical solutions:

1. Tellurium-rich heat treatment process

Studies have shown that in the cadmium-rich state, the entry of Cd atoms will make the tellurium-rich liquid phase formed by the tellurium-rich precipitate defects at high temperature be in a supersaturated state, and then achieve epitaxy on the inner wall of the material, reducing the size of the defect region. Since the number of Te atoms contained in the tellurium-rich precipitate phase is generally greater than the number of Te atoms in the normal crystal material of the same volume, with the continuous entry of Cd atoms, the tellurium-rich phase will fill all the free space, but there is still an excess of Te atoms , because the binding energy of Te atoms and Cd atoms is very large, the reaction will continue, and the volume of the defect region will exceed the volume of the free space, resulting in internal stress in the defect region, and the surrounding materials of the defect will also be affected by internal stress, resulting in Addition of dislocations in the material surrounding the defect. Therefore, in order to avoid the dislocation growth around the defects in the Te-rich precipitate phase caused by the cadmium-rich heat treatment, it is necessary to first exclude the excess Tellurium atoms in the Te-rich precipitate phase defects from the material.

The heat treatment process provides a high-temperature environment in which the atoms in the material can move. At high temperature, the tellurium-rich precipitate phase in the tellurium-rich material will make the material in a tellurium-rich state, that is, the stoichiometric ratio of the material is in a tellurium-rich state, and the gas phase environment corresponding to the equilibrium vapor pressure is a tellurium-rich state (Te ₂ Higher partial pressure, lower Cd partial pressure), if the Te ₂ partial pressure provided by the heat treatment process is lower than the equilibrium vapor pressure of the material, the Te atoms in the material will diffuse to the gas phase, and the Cd partial pressure of the heat treatment system is related to the material chemical The difference in the equilibrium vapor pressure corresponding to the stoichiometric ratio will also cause the Cd atoms to move accordingly. Since the diffusion coefficient of Cd atoms is greater than that of Te atoms, the Cd partial pressure provided by the heat treatment cannot be too high, otherwise the Cd atoms will quickly enter the tellurium-rich precipitate phase (liquid phase at this time) in the CdZnTe material and make it too Saturation leads to epitaxy, and too high Cd pressure will make the surface layer of CdZnTe quickly turn into a cadmium-rich state, which greatly reduces the equilibrium vapor pressure of Te atoms, thereby hindering the diffusion of Te atoms in the material to the gas phase. It can be seen that in order to eliminate the excess Te atoms in the defects of the tellurium-rich precipitate phase, it is necessary to provide a lower gas-phase Cd partial pressure for the tellurium-rich material, and the low Cd partial pressure will fall in the rich Tellurium regions, the so-called tellurium-rich heat treatment. GREENBERG has given the phase diagram of CdZnTe material in a cadmium-rich or tellurium-rich state ^[5] , and the relationship between the Cd partial pressure P _cd and the material temperature T of the CdZnTe material in a positive stoichiometric ratio is,

log P _Cd (atm)=-12.255+0.01187T(°C) (1) The relationship between Cd partial pressure and Cd source temperature T _Cd is,

log P _Cd (atm)＝-5317/T _Cd (K)+5.119, T>594K (2)

The Cd temperature of the tellurium-rich heat treatment is generally chosen to be significantly lower than the Cd source temperature corresponding to the positive stoichiometric ratio. At this time, the equilibrium Te ₂ partial pressure corresponding to the gas phase is relatively high, which is unfavorable for the removal of excess Te atoms in the tellurium-rich material. . However, in the actual tellurium-rich heat treatment process, the system does not provide a high Te ₂ partial pressure, and the heat treatment gas phase environment and the CdZnTe solid phase are actually in an unbalanced non-equilibrium state. In order to slow down the destruction of the non-equilibrium state on the crystal structure of the material surface, a certain amount of powder can be placed in the heat treatment system, and the Te atoms evaporated from the powder can be used to provide a lower Te ₂ partial pressure for the heat treatment system. Under this condition, the excess Te atoms in the Tellurium-rich material will effectively diffuse out, and at the same time, the entry of Cd atoms is not enough to cause supersaturated epitaxy and stress formation in the defect region. The number of Te atoms expelled from the material is positively correlated with the heat treatment temperature, and negatively correlated with the Cd source temperature. By selecting appropriate heat treatment conditions, the number of Te atoms in the tellurium-rich precipitate phase can be controlled to be slightly close to the equivalent space tellurium The number of Te atoms in ZnCd crystals. If the number of Te atoms in the tellurium-rich precipitated phase is excessively discharged, large voids will be left in the precipitated phase region after the next step of cadmium-rich heat treatment process.

2. Cadmium-rich heat treatment process

After the tellurium-rich precipitate defects no longer have an excess of Te atoms, the cadmium-rich heat treatment will be used to reduce or eliminate the precipitate phase defects in the material. The cadmium-rich heat treatment provides the material with a higher Cd partial pressure, and the difference between this and the lower Cd equilibrium vapor pressure corresponding to the material containing the tellurium-rich precipitated phase defects will cause gas-phase Cd atoms to enter the CdZnTe material , so that the Cd content in the tellurium-rich precipitate phase that is in liquid phase at high temperature gradually increases until it is supersaturated, which leads to the epitaxy of the liquid precipitate phase on the crystal surface outside the defect, and makes the defect space in the material smaller. Due to the first step of the tellurium-rich heat treatment, the tellurium-rich precipitate phase will freely transform into CdZnTe crystals in the space occupied by the defect, and this process will no longer generate internal stress, and will no longer be in its surrounding materials. leading to a substantial increase in dislocations. Similarly, the gas-phase environment of cadmium-rich heat treatment is also a non-equilibrium state compared with CdZnTe materials. In order to slow down the damage of this non-equilibrium state to the crystal structure of the material surface, a certain amount of powder can also be placed in the heat treatment system. The evaporated Te atoms provide a lower Te ₂ partial pressure for the heat treatment system.

3. The way of heat treatment process

The heat treatment will be carried out in a high-purity chamber with two temperature zones whose temperature can be independently controlled. Figure 2 is a schematic diagram of the heat treatment device used in the present invention. The heat treatment system consists of a heater 1 , a heat treatment chamber 2 , a sample holder 3 , a sample 4 , a powder 5 and a Cd source 6 . The heat treatment cavity can be a closed quartz ampoule, or a relatively closed graphite cavity placed in a quartz tube cavity (heat treatment process in an open tube mode). The sample is placed in the high-temperature zone of the dual-temperature zone furnace, and the Cd source is placed in the low-temperature zone. The amount of Cd source placed should ensure that it will not evaporate into gas during the heat treatment process, so as to ensure that the Cd partial pressure in the gas phase is controlled by the temperature of the Cd source. The amount of powder determines the depth of the damaged area on the surface of the material, and the removable thickness of the material surface after heat treatment is the reference for selecting the amount of powder.

The two-step heat treatment process can be completed in the same process (that is, without cooling down to room temperature and without opening the cavity) by switching the temperature conditions of the heat treatment, or can be completed in two heat treatment processes as required.

4. Selection of heat treatment temperature

The choice of heat treatment temperature should make Cd and Te atoms have a higher migration rate in the material. Experimental results show that the heat treatment temperature needs to be above 550°C to make the migration of Cd and Te atoms in the material enough to change the tellurium density within a few weeks. Composition and volume of precipitated phases in ZnCd materials. When the heat treatment temperature exceeds 900°C, the dislocation density of the CdZnTe material will begin to increase. Therefore, the heat treatment temperature of the two-step heat treatment process should be selected between 550°C and 900°C. Using such heat treatment temperatures, we performed a tellurium-enrichment heat treatment on CdZnTe wafers (thickness 1mm) using sample temperatures of 550°C, 800°C, and 900°C, and performed a second step of cadmium-enrichment treatment on the material using a sample temperature of 700°C. The results show that the size of the cadmium-rich precipitate phase in the material is significantly reduced, and no dislocation growth occurs in the surrounding material of the precipitate phase defect.

The two-step heat treatment technology for eliminating the defects of the tellurium-rich precipitate phase in the CdZnTe material is a special technology for reducing or eliminating the defect of the tellurium-rich precipitate phase in the CdZnTe material. At the same time, it can suppress the accompanying dislocation propagation effect around the defect while reducing the size of the defect.

Description of drawings

Fig. 1 Changes of surface defects of CdZnTe material containing tellurium-rich precipitate phase defects after cadmium-rich heat treatment, (a) is the surface morphology of the material before heat treatment, and (b) is the surface morphology of the material after heat treatment. The temperature condition of the heat treatment process is 650°C/600°C (sample temperature/Cd source temperature), the heat treatment time is 72 hours, and the surface of the material is corroded by Everson etchant ^[5]

Figure 2 Schematic diagram of the heat treatment system for eliminating defects in the tellurium-rich precipitate phase in CdZnTe materials by two-step method

Detailed ways:

1. Preparation of materials for heat treatment

1) Use an infrared transmission microscope to detect the treated cadmium zinc telluride material, and classify it according to the type of precipitated phase defects contained in the material (tellurium-rich and cadmium-rich precipitated phase defects) and size, and the precipitated phase should be selected for each heat treatment process Samples with approximately the same defect size, and the samples are cleaned as follows;

a) Put the sample into the trichlorethylene melt and heat it to boiling, take the sample out of the melt, replace the trichlorethylene melt and then heat it to boiling, for 3 or more times in a row, take it out and put it into methanol melt Wash in liquid for 3 or more times;

b) Configure Br methanol melt (concentration is 0.5% ~ 1%), put the sample into Br methanol melt and corrode for 10 seconds or more, take it out quickly and put it into methanol melt to wash 3 times or more , and then washed in deionized water for 3 or more times, and then dried with high-purity N ₂ gas before use.

2) Take a certain amount of cadmium zinc telluride polycrystalline material, after cleaning according to the above cleaning method, use a clean agate utensil to pulverize the block material, and obtain a powder whose particle size is less than 1mm;

3) Take some Cd strips with a purity higher than 6N for future use.

4) Process quartz ampoules for heat treatment or make high-purity graphite sample boxes. Before use, quartz ampoules or heat treatment sample boxes must be degassed at a temperature 100°C higher than the temperature of the heat treatment sample (removing the surface of the quartz tube and graphite parts may The presence of adsorbed gas) treatment.

2. Load film

1) Load the sample 4 and the powder 5 into the sample rack 3 and then load the ampoule or the heat treatment sample box 2. The amount of the powder is determined by the control requirements of the heat treatment process for the depth of the surface damage layer. After putting the Cd source 6 into the vacuum (vacuum Temperature is less than 10 ^-4 Pa) and the ampoule inlet is melted and then closed, or the heat-treated sample box is closed with a lid, and the amount of Cd source should ensure that it is not completely vaporized during the entire heat treatment process;

2) Put the ampoule or heat treatment sample box 2 into the dual temperature zone furnace 1, the sample 4 and the Cd source 6 are respectively located in the high temperature zone and the low temperature zone of the dual temperature zone furnace 1. If the heat treatment sample box is used, it is necessary to vacuumize the heat treatment chamber 7 (vacuum degree is less than 10Pa). After the vacuuming is completed, put flowing _N2 gas, and then vacuum again and put flowing _H2 gas.

3. Heat treatment

1) Set the sample temperature and Cd source temperature according to the process conditions of tellurium-rich heat treatment, in order to enable the atoms in the material to migrate effectively during the heat treatment process without causing serious damage to the lattice integrity of the material surface, The temperature of the sample in the heat treatment process should generally be greater than 550°C, but not more than 900°C. The Cd source temperature selected for tellurium-rich heat treatment should be significantly lower than the Cd source temperature calculated according to formulas (1) and (2) (generally less than 50°C). Set the temperature of the heating device of the two-stage temperature zone furnace according to the selected heat treatment temperature, and set the heating rate between 3°C/min and 5°C/min (to make the sample temperature and the Cd source temperature reach the set value at the same time), If the open tube heat treatment process of the heat treatment sample box is adopted, the flowing _H2 gas is switched to the flowing Ar gas after the sample temperature reaches the set value;

2) When the tellurium-rich heat treatment process reaches the tellurium-rich heat treatment time, adjust the sample temperature and the Cd source temperature according to the process conditions of the cadmium-rich heat treatment. The selection range of the sample temperature for the cadmium-rich heat treatment process is also between 550°C and 900°C, and the Cd temperature is significantly higher than the Cd source temperature calculated according to formulas (1) and 3) (generally higher than 50°C) ;

① When the heat treatment time reaches the heat treatment time set by the cadmium-rich heat treatment process, cut off the heating power supply of the furnace heating device in the two-stage temperature zone, so that the heat-treated sample 4 and the Cd source 6 are cooled with the furnace;

② When the temperature in the heat treatment furnace drops to room temperature, take out the quartz ampoule or the heat treatment sample box 2, and take out the treated CdZnTe material.

4. Detection of CdZnTe material performance and judgment of heat treatment effect

1) Use Everson etchant to corrode the surface of CdZnTe material ^[5] . If dislocation corrosion pit clusters as shown in Figure 1(a) are observed on the surface of the material, it indicates that the tellurium-rich heat treatment is not sufficient, and the heat treatment time or the sample temperature of the heat treatment should be increased appropriately;

2) Infrared transmission microscopy is used to detect the precipitated phase defects in the material. If micron-scale bulk defects can still be observed in the CdZnTe material, and there are no corrosion pit clusters in the material, it indicates that Te atoms excluded from the Te-rich precipitation phase defects during the Te-rich heat treatment process have been excessive. At this time, the time of tellurium-rich heat treatment should be appropriately shortened (or the temperature of the sample for tellurium-rich heat treatment should be appropriately reduced); or the time for cadmium-rich heat treatment should be appropriately increased or the temperature of the sample for cadmium-rich heat treatment should be increased appropriately. By adjusting the heat treatment conditions, the number of Te atoms in the tellurium-rich precipitate phase is controlled to be close to the number of Te atoms in the CdZnTe crystal;

3) Use an X-ray diffractometer to measure the twin crystal half peak width of the surface layer of the material, and observe the change of the twin crystal half peak width with the removal depth of the surface layer. The material depth at which the twin crystal half peak width broadens is the thickness of the damaged layer on the surface of the material , by adjusting the heat-treated sample temperature, Cd source temperature and the amount of powder to control the surface damage layer of the material within the allowable range of the subsequent process.

Claims (1)

1. two-step approach eliminates a heat treating method for rich tellurium precipitated phase defect in Cdl-x_Znx_Te, it is characterized in that comprising the following steps:

1) first Cdl-x_Znx_Te is heat-treated under rich tellurium state, tellurium atom excessive in rich tellurium precipitated phase defect is discharged sample; Thermal treatment temp is between 550 DEG C to 900 DEG C, and the setting of Cd source temperature makes sample be in the state of rich tellurium, and the selection of heat treatment time is relevant with thickness of sample with the size of tellurium precipitated phase defect rich in treated material;

2) by rich cadmium thermal treatment process, sample is heat-treated again, Cd atom is made to enter liquid rich tellurium precipitated phase, utilize the process of liquid rich tellurium precipitated phase generation supersaturation extension to reduce the size of rich tellurium precipitated phase defect, and accomplish not produce stress and misfit dislocation to periphery material in heat treatment process; Sample temperature controls between 550 DEG C to 900 DEG C, and Cd source temperature is arranged on and makes sample be in rich cadmium state, and the selection of heat treatment time is relevant with thickness of sample with the size of tellurium precipitated phase defect rich in treated material;

3) the thermal treatment heat treatment technics that adopts a kind of Te dividing potential drop under-balanced state controlled, to reduce in heat treatment process gas phase nonequilibrium situations to the destruction of material surface layer crystal body structure; The Cd point of pressure-controlled that heat treated state is provided by Cd source, place powder in sample area simultaneously, by the quantity of control Cd source temperature and powder, make the Te dividing potential drop that heat treated gas phase remains certain, be in the degree of nonequilibrium situations to slow down in thermal treatment process Te atom in material.