TW201643267A - Method for mixed grade buffer layer epitaxial growth - Google Patents

Abstract

A method of growing an InGaAs metamorphic buffer layer by metal-organic chemical vapor deposition (MOCVD) is provided. The method is characterized in that quantum spectrum response energy absorption of InGaAs extends to 1.0~1.3 eV to thereby apply to quadri-junction solar cells. The method is also effective in reducing the strain otherwise caused by lattice mismatch between adjacent films to thereby apply to III-V multi-junction solar cells.

Description

Hybrid gradient buffer layer epitaxial growth method

The invention relates to a method for epitaxial growth of a mixed graded buffer layer, in particular to a method for epitaxial growth of a mixed graded buffer layer containing both step and linear layers.

At present, the development of a relatively mature three-contact indium gallium phosphide/indium gallium arsenide/gathrene (GaInP/InGaAs/Ge) solar cell has been widely used in the field of concentrating solar energy systems and outer space. In theory, if the solar spectrum is divided into multiple bands, the ratio of the energy bandgap of the material can be adjusted by adjusting the alloy ratio of the material, and the photon energy corresponding to the solar spectrum is increased, so the series connection (tandem) multi-junction solar cells, which use this concept to reduce photon energy loss and improve energy conversion efficiency.

In view of this, adding a junction to the germanium substrate increases the chance of long-band energy absorption; for example, using InGaAsN and InGaAs gradient buffer layer technology, the energy gap can be 1.0. eV~1.3eV. Among them, the indium-arsenide gallium (InGaAsN) electronic materials have poor characteristics, low electron mobility and short carrier life, resulting in a short diffusion length of a few carriers is not conducive to photocurrent extraction; in comparison, Indium gallium arsenide gradient buffer layer technology can not only solve the limitation faced by multi-junction of indium gallium arsenide, but also achieve the effect of absorbing long-band.

It is worth noting that the Grad Buffer Layer is grown, and each layer of the gradient layer must be considered: 1. Indium composition ratio, 2. Film thickness, 3. Stress, etc. Mainly due to the indium atom of indium gallium arsenide changing the lattice state, and the high indium composition ratio is easy to cause epitaxial defects; similarly, the film thickness still needs to be controlled within the critical thickness to avoid film cracking caused by excessive stress in the film (Crack ). Indium gallium arsenide (In _1-x Ga _x As) graded layer grows on the germanium substrate, usually requires a lattice mismatch between the film and the germanium substrate of less than 2%, and it is easier to obtain a high quality film, but this limit is relatively affected. The material extends the long-band energy absorption opportunity. Therefore, the design of the indium gallium arsenide grade layer condition is important.

In general, indium gallium arsenide gradient buffer layer growth technology covers step by step (step by step), linear growth method (linear), although both methods have improved material absorption of long-wavelength photon energy, but due to growth mechanism Different, the film quality obtained is quite different from the surface state.

The growth of the indium arsenide crystal lattice deformation buffer layer, the lattice constant of which changes with the increase of the indium composition; in other words, each layer of the film must have a compressive stress (Compressive strain) due to the mismatch of the lattice constants. When the stress is greater than the critical value, the Misfit Dislocation will cause the non-radiative recombination phenomenon, and the energy conversion efficiency cannot be improved.

"Step growth method" refers to the stepwise increase of the indium composition of each layer of indium gallium arsenide, and the completion of a single layer of thin film deposition and then adjust the growth parameters, and then continue the film deposition when the conditions are met, and so on; It is more time consuming, and the interface between the films is not easy to obtain a flat surface. The photograph of the cross section of the graded buffer layer epitaxial using the step growth method under a transmission electron microscope is shown in Fig. 1. It can be seen from Fig. 1 that when the step growth method is used, there is a high defect density at the junction between the indium gallium arsenide films. The photo of the surface of the epitaxial buffer layer using the step growth method under the optical microscope (magnification 50 times) is shown in Fig. 2. It can be observed from Fig. 2 that the surface crossing line (Crosshatched) is obvious.

"Linear growth method" means that the growth process of the graded layer is carried out in a continuous and uninterrupted manner, and the flow rate of the three-group reaction source increases linearly with the growth temperature of the epitaxial growth. The biggest disadvantage of this growth mode is that it is difficult to control the composition of the indium of each graded layer. Since the indium atom is significantly affected by the temperature of the reaction chamber when the growth temperature is increased, the elemental reaction is performed in an unstable environment, and it is difficult to obtain a good film quality. The photograph of the transverse section of the graded buffer layer epitaxial using the linear growth method under a transmission electron microscope is shown in FIG. It can be seen from Fig. 3 that when the linear growth method is used, it is difficult to obtain good film quality.

Therefore, the use of step growth method or linear growth method to construct sub-battery conditions is theoretically and technically feasible, but how to obtain good film quality to facilitate the subsequent deposition of other materials has become an important issue.

In order to solve the conventional GaAs gradation buffer layer growth technology, each layer of film must have compressive stress due to lattice constant mismatch. When the stress is greater than the critical value, the defect will cause non-radiative recombination phenomenon, resulting in energy conversion. The problem that the efficiency cannot be improved, the present invention provides a method for epitaxial growth of a hybrid graded buffer layer.

In order to achieve the above and other objects, a method for epitaxial growth of a hybrid graded buffer layer of the present invention comprises: A. placing a germanium substrate in a reaction chamber of an organometallic chemical vapor deposition system, and introducing hydrogen gas (H) ₂ ) heating to remove the native oxide on the ruthenium substrate; B. introducing a gas containing arsenic as a group V material into the reaction chamber, and introducing a gas containing indium element and a gas containing gallium as a gas Group III materials; C. The first step of growth of indium gallium arsenide (In _1-x Ga _x As) on a germanium substrate by organometallic chemical vapor deposition, maintaining a growth temperature of one during growth The initial temperature, and maintain the ratio of the five elements to the three elements in the gas (V/III ratio) as an initial ratio; D. the growth of the first step by the organometallic chemical vapor deposition method by arsenic a first linear layer composed of indium gallium, during which the growth temperature is gradually increased from the initial temperature to a second temperature, and the ratio of the five elements to the three elements in the gas is gradually increased from the initial ratio Lower to one a second ratio; and E. repeating step C and step D a plurality of times to sequentially grow the second step layer, the second linear layer, the third step layer, the third linear layer, and the fourth step on the first linear layer a hierarchy, a fourth linear layer, a fifth step hierarchy, a fifth linear layer, a sixth step hierarchy, and a sixth linear layer, thereby forming a gradation layer composed of six step levels and six step levels, wherein In the process of repeating steps C and D, the previous second temperature system is used as the subsequent initial temperature, and the previous second ratio is taken as the subsequent initial ratio.

In the above method, in the growth of the graded layer, the ratio of the group V element to the group III element in the gas is reduced from 16.1 to 15.1, and the growth temperature is increased from 550 ° C to 650 ° C.

The above method further comprises: F. growing a stress relaxation layer composed of indium gallium arsenide on the sixth linear layer by an organometallic chemical vapor deposition method, and controlling the growth temperature to be 620-640 ° C during the growth process Controlling the ratio of the five-group to three-group ratio (V/III ratio) of the gas into 14.1 to 16.1; and G. forming a virtual alloy of indium gallium arsenide on the stress-relieving layer by organometallic chemical vapor deposition In the buffer layer, during the growth process, the controlled growth temperature is 620-650 ° C, and the ratio of the V group to the tri-family element (V/III ratio) in the controlled gas is 39.8-41.8.

In the above method, in the steps A to G, the pressure in the reaction chamber is between 30 and 50 mbar.

In the above method, in step C, the growth time of the first to sixth step levels is 180 to 205 seconds.

In the above method, in the step D, the growth time of the first to sixth linear layers is 30 seconds.

In the above method, in the step D, the difference between the initial temperature and the second temperature is 10 °C.

In the above method, in step D, the difference between the initial ratio and the second ratio is between 0.1 and 0.3.

The above method, wherein the arsenic-containing gas system is selected from the group consisting of arsine (AsH ₃ ) and tert-butyl arsenic (TBAs).

In the above method, the gas system containing the indium element is trimethyl indium (TMIn), and the gas system containing the gallium element is trimethylgallium (TMGa).

The method for epitaxial growth of the hybrid graded buffer layer of the present invention can achieve the effect of extending the energy absorption of the long-wavelength band and improving the stress in the film compared with the prior art.

In order to fully understand the objects, features and advantages of the present invention, the present invention will be described in detail by the following specific embodiments and the accompanying drawings.

The invention mainly grows a metamorphic buffer layer of indium gallium arsenide by a metal organic chemical vapor deposition system (MOCVD). The growth technology enables the quantum-spectrum energy absorption of indium gallium arsenide to be extended to 1.0-1.3 eV, which can be used as a four-junction solar cell structure, and at the same time improve the lattice mismatch between films (Lattice mismatch). Stress, so good film quality can be obtained, suitable for the development of three-five multi-junction solar cells.

The main feature of the present invention is to combine the "step growth method" and the "linear growth method" to form a "hybrid gradient buffer layer epitaxial growth technique", which uses an indium gallium arsenide gradient buffer layer formed by lattice deformation. It can extend the absorption of long-band energy and improve the stress in the film.

The method for epitaxial growth of the hybrid graded buffer layer of the present invention comprises the following steps:

Step A: The ruthenium substrate is placed in a reaction chamber of an organometallic chemical vapor deposition system, and hydrogen (H ₂ ) is introduced to be heated to remove the native oxide on the ruthenium substrate. Among them, it is preferred to heat the 680-780 ° C for more than five minutes to remove the native oxide on the ruthenium substrate, and the optimum parameter condition is 750 ° C for 10 minutes.

Step B: In the reaction chamber, a gas containing arsenic is introduced as a Group 5 material, and a gas containing an indium element and a gas containing a gallium element are introduced as a group III material. Among them, arsine (AsH ₃ ) may be used as the Group 5 material, and tert-butyl arsenic (TBAs) may be used instead of the arsine as the Group 5 material, but is not limited thereto. At low temperatures, TBAs have better cleavage efficiency than AsH ₃ . Therefore, when the mixed graded buffer layer epitaxy of the present invention is prepared at a relatively low growth temperature, TBAs are preferably used.

Step C: growing a first step layer composed of indium gallium arsenide (In _1-x Ga _x As) on the germanium substrate by an organometallic chemical vapor deposition method, and maintaining the growth temperature at an initial temperature during the growth process. And maintaining the ratio of the group V element to the group III element (V/III ratio) in the gas to be introduced is an initial ratio.

Step D: growing a first linear layer composed of indium gallium arsenide on the first step by organometallic chemical vapor deposition, and gradually increasing the growth temperature from the initial temperature to a second during the growth process. The temperature is gradually reduced from the initial ratio to a second ratio by the ratio of the five elements to the three elements in the gas.

Step E: repeating step C and step D a plurality of times to sequentially grow the second step layer, the second linear layer, the third step level, the third linear layer, the fourth step level, and the first linear layer a four-linear layer, a fifth-step hierarchy, a fifth linear layer, a sixth-step hierarchy, and a sixth linear layer, thereby forming a gradation layer composed of six step levels and six step levels, wherein the steps are repeated a plurality of times In the process of C and step D, the previous second temperature is taken as the subsequent initial temperature, and the previous second ratio is taken as the subsequent initial ratio.

The above steps C~E are performed by sequentially depositing a plurality of layers of indium gallium arsenide thin films on the germanium substrate to form the graded layer of the present invention. During the growth of the graded layer, the ratio of the five elements to the three elements in the gas can be reduced from 16.1 to 15.1; the growth temperature can be increased from 550 ° C to 650 ° C. It is worth noting that the different growth temperatures and indium composition ratios between the layers of the graded layer are like increasing the ratio of indium composition layer by layer. This method can reduce the cross-hatching problem caused by lattice mismatch.

In order to further reduce the problem of lattice mismatch, the method for epitaxial growth of the hybrid graded buffer layer of the present invention may further comprise the following steps:

Step F: growing a stress relaxation layer composed of indium gallium arsenide on the sixth linear layer by an organometallic chemical vapor deposition method, and controlling the growth temperature to be 620-640 ° C during the growth process, and controlling the gas into the gas The family-to-three ratio (V/III ratio) is 14.1 to 16.1. Step F is to reduce the internal stress of the graded layer to affect the subsequent growth thinness. Therefore, the growth of a “overshoot layer” with a high proportion of indium on the graded layer helps to release the stress in the film to avoid Affects subsequent film growth results.

Step G: growing a virtual buffer layer composed of indium gallium arsenide on the stress relaxation layer by organometallic chemical vapor deposition, and controlling the growth temperature to be 620-650 ° C during the growth process, and controlling the five gases in the gas. The ratio of elements to tri-family elements (V/III ratio) is 39.8~41.8. The virtual buffer layer of the present invention grows on the stress relaxation layer and adjusts the indium composition ratio to control the energy gap energy range between 1.0 and 1.3 eV. At this time, the surface state of the film is flat and the energy of the long band is absorbed. The effect. Therefore, this layer is also referred to as a single crystal substrate, also referred to as a virtual substrate (Pseudo Substrate).

In the above steps A~G, in general, the low pressure helps to increase the indium incorporation rate, but the pumping efficiency factor of the restricted organometallic chemical vapor deposition system is generally controlled at 30~. Range of 50 mbar.

Example

The mixed graded buffer layer epitaxy of this embodiment is prepared by the above steps A~G, and the detailed process parameters are shown in Table 1: Table 1:

In Table 1, S1~S6 represent the first step to the sixth step, respectively; L1~L6 represent the first linear layer to the sixth linear layer, respectively. During the growth of the graded layer, the ratio of the five elements to the three elements in the gas was reduced from 16.1 to 15.1, and the growth temperature was increased from 550 °C to 650 °C. The growth time of the first to sixth steps is 180 to 205 seconds. The growth time of the first to sixth linear layers is 30 seconds. In step D, the difference between the initial temperature and the second temperature is 10 ° C, that is, during the growth of each linear layer, the temperature is gradually increased by 10 ° C. In step D, the difference between the initial ratio and the second ratio is between 0.1 and 0.3, that is, during the growth of each linear layer, the ratio of the five groups to the three groups (V/III ratio) is gradually introduced into the gas. Decrease by 0.1~0.3.

In the growth process of the graded layer, the stress release layer and the dummy buffer layer of the present embodiment, the relationship between the growth temperature and the growth time is as shown in FIG. In Fig. 4, in order to be able to clearly show the linear temperature rise process of the linear layer, the growth time of each linear layer (L1 to L6) is deliberately enlarged.

The Grade layer of this embodiment utilizes hydrogen arsenide (AsH ₃ ) as a Group 5 material; and uses trimethyl arsenic (TMIn) and trimethylgallium (TMGa) as a group III material. The growth temperature control range is 570~630°C, and the V/III ratio is adjusted from 16.1 to 15.1 as the growth temperature increases. The growth of the growth temperature does not turn off the TMIn/TMGa flow. To adjust the indium composition to complete linear growth, to avoid the instability of the flow changes affecting the surface state of the film, in addition to increasing the incorporation of indium and reducing stress.

The Overshoot Layer of the present embodiment is susceptible to subsequent films due to stress in the graded layer, such as an epitaxial defect. Therefore, the stress-relieving layer is designed, and the indium composition is increased by 12% compared with the graded layer to release the stress of the graded layer, thereby avoiding the influence of subsequent film growth.

The virtual buffer of the present embodiment has a growth temperature of 620 ° C, a growth rate of about 3 μm/hr, a growth thickness of about 1.0 μm, a five-group to three-group ratio (V/III ratio) of 40.8, and an indium composition of 20%. (In composition), a flat film can be obtained, which is suitable for use as a substrate for subsequent film deposition, and is therefore also referred to as a "virtual substrate."

The photograph of the transverse section of the epitaxial gradient layer of the hybrid type gradient buffer layer of this embodiment under a transmission electron microscope is shown in FIG. It can be seen from Fig. 5 that the lateral defects are less than 0.3 μm and are suppressed in the graded layer, and the film state thereon is flat and defect-free. The photograph of the surface of the mixed graded buffer layer epitaxial layer of this embodiment under an optical microscope (magnification 200 times) is shown in Fig. 6. From Fig. 6, it was observed that the surface was not found to have a cross hatched line.

It can be understood from the above examples and the test results thereof that the method for epitaxial growth of the mixed graded buffer layer of the present invention can effectively improve the stress generated by the lattice mismatch between the films, so that good film quality can be obtained. Suitable for the development of three-five multi-junction solar cells.

The invention succeeds in gradually increasing the proportion of indium composition by the "mixed graded retardation epitaxial growth technology", increasing the solar spectrum absorption range of indium gallium arsenide, and simultaneously reducing the influence of the internal compressive stress of the material to obtain better film quality and improving the film quality. Component design flexibility.

In addition, by the concept of the "hybrid gradient retardation epitaxial growth technology" of the present invention, another component of the component material developer, such as a molecular beam epitaxy (MBE), is added, and the breadth of the application surface is more than In addition to the development of solar cells, it is also possible to further promote the MQW Strain engineering and laser diode epitaxy technology of the GaN series.

The invention has been described above in terms of the preferred embodiments, and it should be understood by those skilled in the art that the present invention is not intended to limit the scope of the invention. It should be noted that variations and permutations equivalent to those of the embodiments are intended to be included within the scope of the present invention. Therefore, the scope of protection of the present invention is defined by the scope of the patent application.

no

Fig. 1 is a photograph of a cross section of a graded buffer layer epitaxial layer using a step growth method under a transmission electron microscope. Fig. 2 is a photograph of a surface of a graded buffer layer epitaxial layer using a step growth method under an optical microscope. 3 is a photograph of a cross section of a graded buffer layer epitaxial layer using a linear growth method under a transmission electron microscope. FIG. 4 is a relationship between growth temperature and growth time of an embodiment of the present invention. FIG. Photograph of the hybrid graded buffer layer epitaxial cross section of the embodiment under a transmission electron microscope. FIG. 6 Photograph of the surface of the mixed graded buffer layer epitaxial layer of the present embodiment under an optical microscope.

Claims (10)

A method for epitaxial growth of a mixed graded buffer layer comprises: A. placing a germanium substrate in a reaction chamber of an organometallic chemical vapor deposition system, introducing hydrogen gas (H ₂ ), and heating to remove the native layer on the germanium substrate Oxide; B. In the reaction chamber, a gas containing arsenic is introduced as a Group 5 material, and a gas containing an indium element and a gas containing a gallium element are introduced as a group III material; C. an organic metal chemical gas The phase deposition method grows the first step of the indium gallium arsenide (In _1-x Ga _x As) on the germanium substrate. During the growth process, the growth temperature is maintained at an initial temperature, and the five gases in the gas are maintained. The ratio of the element to the group of three elements (V/III ratio) is an initial ratio; D. the first linear layer composed of indium gallium arsenide is grown on the first step by organometallic chemical vapor deposition. During the growth process, the growth temperature is gradually increased from the initial temperature to a second temperature, and the ratio of the five elements to the three elements in the gas is gradually reduced from the initial ratio to a second ratio; and E. Multiple steps C and Step D, sequentially growing the second step layer, the second linear layer, the third step layer, the third linear layer, the fourth step layer, the fourth linear layer, and the fifth step layer on the first linear layer. a fifth linear layer, a sixth step hierarchy, and a sixth linear layer, thereby forming a gradation layer composed of six step levels and six step levels, wherein in the process of repeating steps C and D multiple times, before The second second temperature is used as the next initial temperature, and the second ratio is the initial ratio. The method of claim 1, wherein during the growth of the graded layer, the ratio of the group V element to the group III element in the gas is reduced from 16.1 to 15.1, and the growth temperature is increased from 550 ° C to 650 ° C. The method of any one of claims 1 or 2, further comprising: F. growing a stress relief layer composed of indium gallium arsenide on the sixth linear layer by organometallic chemical vapor deposition, growing In the process, the controlled growth temperature is 620~640 °C, and the V/III ratio of the controlled gas is 14.1~16.1; and G. is deposited by the organometallic chemical vapor deposition method. The growth of the virtual buffer layer composed of indium gallium arsenide is controlled to grow at a temperature of 620 to 650 ° C. The ratio of the five elements to the three elements (V/III ratio) in the controlled gas is 39.8~ 41.8. The method of claim 3, wherein in the steps A to G, the pressure in the reaction chamber is between 30 and 50 mbar. The method of claim 3, wherein in step C, the growth time of the first to sixth step levels is 180 to 205 seconds. The method of claim 3, wherein in step D, the growth time of the first to sixth linear layers is 30 seconds. The method of claim 3, wherein in step D, the difference between the initial temperature and the second temperature is 10 °C. The method of claim 3, wherein in step D, the difference between the initial ratio and the second ratio is between 0.1 and 0.3. The method of claim 1, wherein the arsenic-containing gas system is selected from the group consisting of arsine (AsH ₃ ) and t-butyl arsenic (TBAs). The method of claim 1, wherein the gas system containing the indium element is trimethyl indium (TMIn), and the gas system containing the gallium element is trimethylgallium (TMGa).