DE2117472B2 - Process for the production of a three-layer semiconductor body - Google Patents

Description

The invention relates to a method for producing a three-layer semiconductor body, the layers of which are enitaxially grown on a gallium arsenide substrate, which consist of two layers wide energy band gap with opposite conductivity type and one in between Layer of narrower energy band gap exist and bring them into contact with each other of the substrate with molten solutions containing gallium, aluminum and arsenic different aluminum content and different doping and controlled cooling to be deposited.

Through more recent areas (see Journal of Applied Physics, 41, [1970], 150), according to which the threshold current density for stimulated emission in semiconductor lasers from earlier values of the order of magnitude of 25,000 A / cm ² or greater to values down to 6000 A / cm ² could be reduced at room temperature, the interest in semiconductor lasers that can be operated at room temperature was stimulated again. The low threshold values facilitate operation at room temperature by reducing overheating of the device

These lower threshold densities are achieved in a multilayer semiconductor body, a so-called single heterostructure (EH), which has adjacent zones of a material with a wide band gap (e.g. p-conducting Ga, _, A _x As) and a material with a narrow band gap (e.g. GaAs) that are separated by a heterojunction. A heterojunction is defined as the boundary layer between a zone with a small band gap and a zone with a wide band gap and is further characterized by nn, pp, np or pn, depending on which type of charge carrier represents the majority charge carrier on both sides of the junction. In the case of an EH diode, a pn junction is arranged in the zone of small bandgap so that an intermediate zone is present between the pn junction and the heterojunction in which radiative recombination of holes and electrons takes place when the diode is forward biased. It is assumed that the confinement of electrons and photons to the intermediate zone are responsible for the low threshold values. A prerequisite for these advantageous results is that the thickness of the intermediate zone is less than the diffusion length of the injected charge carriers and in any case less than approximately 2 μm in the EH element. Attempts have therefore been made to develop a method suitable for growing such structures, in particular the thickness of the intermediate zone should be controlled. For example, the production of EH laser diodes takes place in the epitaxial process from the liquid phase, which is carried out in a tilting device in which a substrate carried by a holder is arranged over a hole containing the molten solution. The otherwise movable substrate holder is provided with means to remove oxide impurities from the surface of the starting solution before the growth process. For example, an η-conductive gallium arsenide substrate is applied to a starting solution containing p-type dopants and is epitaxial. Causes growth. In the process, an epitaxial p-line grows

tend thin film on the substrate, during the growth process and also afterwards the dopant diffuses into the η-conductive substrate, so that a pn junction arises in the substrate.

While the EH diode represents a significant advance in the effort to lower thresholds and therefore represents semiconductor lasers that can be operated at room temperature a further reduction in the threshold current density in a so-called double hetero structure (DH) achieved in which the mentioned recombination intermediate zone of small band gap not only as in of the EH diode is adjacent to a zone with a wider band gap on one side, but (according to Soviet Physics-Semiconductors, Vol. 2 [1969] 1289 and Kristall u. Technik, Vol. 4 [1969] 495) wider between two zones Band gap and opposite conduction type is arranged.

However, the reduction in the threshold current density for stimulated emission that can be achieved is not yet particularly satisfactory. For example, the DH semiconductor injection laser described in "Soviet Physics-Semiconductors" loc. Cit., In which the thickness of the intermediate layer is between 5 and 15 μm, shows current threshold values for stimulated emission between about 2000 A / cm ² and more than 3000 A / cm ² at the temperature of liquid nitrogen (77 ° K). Since these thresholds increase with the operating temperature of the laser, operation at room temperature would require thresholds that are at least an order of magnitude higher.

The object of the invention is therefore to specify a method with which three-layer semiconductor components of the type described in the introduction can be produced so that they can be used in their use significantly lower than DH semiconductor injection lasers Show current thresholds for stimulated emission than before.

According to the invention, this object is achieved for the method described in the introduction in that the The narrow band gap layer (the intermediate layer) by controlling its growth temperature and the cooling period during its deposition is limited to a thickness below 2 μm will.

Due to the inventive limitation of the thickness of the intermediate layer with a narrow band gap to values below 2 / in, preferably below 1 / in, it is possible to achieve a threshold current density for stimulated emission which is only 2000 to 3000 A / cm ² at room temperature.

Furthermore, these semiconductor structures with or without doping are also suitable as optical waveguides, in which the light propagates in the narrow band gap layer and through the various Refractive indices of the adjacent layers of wide band gap is performed.

The multilayer semiconductor, which has alternating layers of wide and narrow bandgap, is produced in an epitaxial process in which separate starting solutions are used one after the other brought into contact with one of the specified III-V compounds in a precisely controlled manner that epitaxial growth takes place in each case and the individual layers are thus obtained.

A tilting or sliding device can be used for this purpose in order to successively bring the solutions into contact with the substrate. Definitely However, it has been shown that a temperature gradient in which the solutions above a lower Temperature as below in the vicinity of the substrate is desirable to have the thin intermediate layer narrow band gap that is less than about 2 / in thick.

ίο For growing a DH laser diode with, for example a npp layer sequence is a first solution of Ga-Al-GaAS, which is saturated with As and a Contains η-type dopant, brought into contact with an n-type GaAs substrate, in order to place an epitaxial thereon To let η-conductive Ga ^ Al ^ As layer grow with a wide band gap. Then becomes a second solution of GaAs in Ga saturated with As and preferably a compensating dopant contains, brought into contact with the substrate to form a p-type interlayer with a narrow band gap to let grow, which is essentially compensated. Finally, there is a third solution of Ga-Al-GaAs, which is saturated with As and contains a p-type dopant, in contact with the substrate

»5 brought in order to let a p-type Ga, _ _x Al _x As layer grow and thereby complete the DH structure.

The invention is described below with reference to the drawing; it shows

Fig. 1 is a view, partly in section, of a tilting apparatus for carrying out the invention Procedure,

FIG. 2 is a plan view of the shuttle and carriage of the apparatus according to FIG. 1,

3 to 7 cross sections through a DH semiconductor body in successive stages of manufacture and

Fig. 8 is a view, partly in section, of further apparatus for carrying out the method.

The crystal growing apparatus of FIGS. 1 and 2 has growth tube 11, typically made of quartz glass, for the crystal and has a gas inlet 12 and a gas outlet 13 for insertion and extraction

♦ 5 of gases. A shuttle 14 is also provided The shuttle 14 has two stops 25 and 26 and a recess for securely holding the substrate A movable solution holder 15 is arranged in the shuttle, and the bores 16a to 16c close

Has recording of starting solutions. The Hai sion 15 is also provided with grooves 18 to oxides and similar solid contaminants from the bottom surface of each of the starting solutions in the well egg 16a to 16c to be removed. The device also includes a thermocouple bore 20 and a therm moelement 21 to measure the temperature of the device. The tube 11 is, as shown, ii a furnace 22 is introduced which has an observation opening 23. The oven 22 is on a fork 2 '

stored, which equips a tilting of the cultivation tube 11 ge.

To produce a laser diode with Doppelhe tero structure according to the present method win for example from an η-conductive gallium arse

Assumed nid substrate, which is commercially available. For the purpose envisaged here, the substrate should have a charge carrier concentration of 10 ¹⁷ to 10 "elections / ccm. Wen

a material with less than about 10 ¹⁷ electrons / ccm is selected, the conductivity would possibly be insufficient in laser continuous wave operation without overheating. The maximum charge carrier concentration is dictated by practical considerations. The material is then lapped and cleaned in a known manner in order to obtain a clean surface. The section through a typical substrate 31 is shown in FIG. 3.

An apparatus of the type shown in FIGS. 1 and 2 with a growing quartz tube is then used and a carbon boat selected. To prepare the starting molten solutions I, II and III, which consists of gallium, aluminum, gallium arsenide in varying contents and corresponding dopants exist, known (overdosed) amounts of solid, commercially available gallium arsenide (99.9999% purity) known amounts of gallium (99.9999% purity) are added and this Mixtures in a pure hydrogen atmosphere heated to a sufficiently high temperature to completely remove the gallium arsenide to solve. The solution is then cooled, and the necessary amounts of aluminum and dopant are added so that melt solutions of the desired composition are obtained subsequent heating. The gallium, gallium arsenide, and aluminum amounts give through from the gallium-aluminum-arsenic three-phase diagram and the desired bandgaps following considerations, while the amount of dopant due to the doping (and therefore the resistance) is specified, which is desired in the various zones of the finished semiconductor component is.

Specifically, each melt solution contains an excess of GaAs and, by way of illustration, the following Components:

(I) Melt solution I contains Ga, Al and GaAs with 1.5 to 2.5 mg Al / g Ga and a dopant from η type (e.g. 10 to 20 mg Sn / g Ga, 2 to 3 mg Si / g Ga, or 0.1 to 1 mg Te / g Ga);

(H) Melt solution II contains Ga and GaAs or Ga, Al and GaAs with compensated doping (e.g.

1 to 4 mg Si / g Ga or about 1 mg Si and 1 to

2 mg Zn per gram of Ga) and provided that in the latter case the melt solution II is a lower percentage of Al than either of solutions I. or III contains; and

(III) Melt solution III contains Ga, Al and GaAs with 2 to 3 mg Al / g Ga and a dopant from p-type (e.g. 3 to 5 mg Zn / g Ga).

The components of the melt solutions I, II and III are introduced into the bores 16a, 166 and 16c of the holder 15, which initially abuts the wall 14a of the boat 14. The substrate is inserted into the recess in the bottom of the boat 14 and the device is purged with nitrogen. Purified hydrogen is then admitted and the temperature is increased to 700 to 1100 ° C. to obtain the molten solution, depending on the composition of the selected molten solution. After the maximum temperature (eg 870 ° C.) has been reached, cooling is carried out at a predetermined rate (eg 1 to 6 ° C./minute). When the desired first tilting temperature (for example 850 ° C) is reached, the device is set in motion by tilting the boat in the direction of arrow 1 so that the front edge 15a of the solution holder 15 abuts against the end wall 14 d of the boat 14 and the solution I is above the substrate 31. Simultaneously with the sliding movement of the holder 15, the grooves 18 remove the oxide layer from the underside of the molten solution I. The cooling program is continued. The thin film 32 of η-conductive Ga _1-1 Al _x As, which thereby grows on the η-conductive GaAs substrate 31, is shown in FIG.

As the cooling continues, the device reaches

ίο a second tilt temperature (e.g. 830 ° C), whereupon the shuttle is tilted in the opposite direction (arrow 2). This hits the edge 15ö the bracket 15 at the stop 26. At the same time, the grooves 18 remove the oxide layer from the underside the melt solution II, which is now above the substrate 31. By cooling down continuously during a period of time which depends on the desired thickness of the intermediate layer becomes a p-type GaAs intermediate layer 33 with a narrow band gap

ao let grow up (Fig. 5). In this way, a pn heterojunction 32a is created at the interface between layers 32 and 33.

As the cooling continues, a third tilting temperature is finally reached. Then the pipe

as 11 rotated about its longitudinal axis until the holder 15 strikes against the side wall 14 b of the shuttle 14. The boat is then tilted in the direction of arrow 3 until the wall 156 of the holder abuts against the hand 14c of the boat, the grooves 18 having removed the oxide layer of the molten solution III on the underside, which is now above the substrate 31. The growth process is then continued until a p-type Ga x Al _y As _ zone 34 of a wide band gap (FIG. 6) is raised in the desired thickness.

It has been shown that the observation opening 23 has a temperature gradient for the tilting process in a direction substantially perpendicular to the growth surface as a result of out of the window

The heat flow emerging from the opening is generated. The molten solution is therefore hotter in its lower part at the substrate than in its upper part, which causes the molten solution to settle more strongly in its upper part (namely on floating nuclei), so that intermediate layers as thin as can be grown very easily these are preferred for low threshold stimulated emission. In this way, layers with a thickness of less than 1 μm were produced.

The apparatus according to FIG. 8 has no observation opening, but an auxiliary heater in order to generate a controllable temperature gradient. The apparatus comprises a growth tube 111 with a gas inlet 112 and a gas outlet 113 and a molten solution holder 115 which has a cylindrical cross section and a plurality of bores 116a to 116a ¹ for receiving the starting melts. A substrate holder 114 is slidably inserted into a channel in the solution holder 115. The substrate

Bracket 114 has a recess for receiving a substrate 119. Bracket 114 abuts on the left against a stop 117. It should be noted that the grooves 18 of the device shown in Fig. 1 are omitted because the substrate holder 114 is made of graphite

Phite is manufactured with a relatively rough upper side, which is able to remove the oxide layer from the underside of the melt solutions 116a to 116a ^1. The tube 111 also has an opening 120 through which a thin

RB Ii C

Tube 121 is inserted into thermocouple bore 122 in the base of the molten solution holder 115. A thermocouple 123 is located in the tube 121. A rod 125 or the like is inserted through the opening 124, in order to move the solution holder 115 relative to the substrate holder 114. The tube 111 Sst, in an oven 126 (which has main heating elements 127 and 128) by packing elements 129 as shown and 130 attached. The auxiliary heater 131, which generates the mentioned temperature gradient, is between the underside of the solution holder and the inside of the furnace wall. The observation port 23 of the device shown in Fig. 1 is missing here. In the apparatus of FIG. 8, the Compositions of melt solutions I, II and III and the process program are used as this has been described in connection with the apparatus of FIG. In the present case, however, the temperature gradient, which is advantageous for the growth of the thin intermediate layers, generated by the auxiliary heater 131. At each, the tilt temperatures of the previous procedure corresponding temperature, the rod 125 is moved in order to move the holder 115 and thereby to bring the melt solution I, II and III over the substrate 119 one after the other. The solution holder but can also be arranged in a stationary manner if the substrate holder can be moved relative to it.

The bracket 115 (Fig. 8) has a fourth hole, the hole 116 <i for receiving a Melt solution IV, the Ga, GaAs in excess and a p-dopant (e.g. 10 mg Zn / g Ga or 2 to 3 mg Ge / g Ga). This solution produces a p-type GaAs layer 35 (FIG. 7), on which thereafter an ohmic contact 37 can be attached in a known manner. This creates the difficulty resolved the need to produce such a contact on the mixed crystal layer 34. An ohmic Contact 36 on GaAs substrate 31 can also be formed in a known manner.

The space between the surface 114o of the substrate holder 114 and the surface 115a of the solution holder 115 is approximately 0.1 to 0.3 mm, but only approximately 0.02 to 0.05 mm for the surface 115b. Due to the smaller distance at the surface 115fa the last solution is stripped from the surface of the substrate, so that no further growth occurs when the substrate is withdrawn from the melt solution IV b and moved into the area of the surface 115th Finally, the brackets can be removed from the tube by opening the conical connection 140.

The shape of the bores 116a to 116 <f, which widens in a funnel-like manner downwards, and the space mentioned allow a continuous layer of molten solution to be maintained over the seed once the first enamel solution has been put in place. Although this feature is not strictly necessary it has been shown that structures with better consistency and good quality are achieved, if this gap and this shape of the holes are used.

example

This example is the manufacture of a DH semiconductor injection laser low threshold according to the present method using the apparatus of FIG.

A silicon-doped gallium arsenide substrate (size about 6.25 mm X 12.5 mm X 0.5 mm) with about 4 x 10 ¹⁸ electrons / ccm and with an end face perpendicular to the (100) direction and made of a commercially available Material was chosen as the substrate. The substrate was lapped with 305-carbonind, rinsed with deionized water, and etch polished with a bromine-methanol solution to remove surface damage.

ίο Then there were four solutions in the following way prepared. First, the following amounts of material were weighed out.

For melt solution I: 1 g Ga, 100 mg GaAs (undoped), 2 mg Al and 15 mg Sn.

For molten solution II: 1 gGa, 100 mg GaAs (undoped) and 1 mg Si.

For melt solution III: 1 g Ga, 50 mg GaAs (undoped), 3 mg Al and 5 mg Zn.

For melt solution IV: 1 g Ga, 75 mg GaAs (undo-

ao animals) and 32 mg Ge.

The gallium and the gallium arsenide were briefly preheated to 900 ° C under H _r atmosphere in a graphite solution holder for each solution. The substrate and the four prepared melt dissolving

5 sets of gallium and gallium arsenide were placed in their respective bores in the apparatus of FIG. The rest of the balanced components were then placed in the correct bores with the premixed gallium and gallium arsenide and mechanically pushed under the surface of the liquid gallium to ensure good contact during subsequent heating. The holding fixture was then inserted into a quartz glass tube (Fig. 8). The air was then flushed out with hydrogen for 10 minutes. The tube was then inserted into the 870 ° C oven. The auxiliary heater, which consisted of a single 62 cm long 0.5 mm nickel-chromium wire loop and was fed with 20 volts alternating current (Fig. 8), was switched on during this process. The measured temperature was allowed to rise to about 870 ° C., and then a cooling rate of about 3 ° C./minute was set. At 850 ° C., the solution holder was moved so that the melt solution I came into contact with the substrate. A mechanical vibrator kept the solution in gentle agitation while cooling it to 830 ° C. At 830 ° C., the solution holder was shifted so that the melt solution II covered the substrate. Melt Solution II remained in this position, vibrating for about 15 seconds. The solution holder was then shifted again so that the substrate came under the melt solution III. It was held in this position for 30 seconds (with vibration). The solution holder was then moved again so that the substrate came under the molten solution IV where it was held for 60 seconds (with vibration). Then the substrate holder 114 was moved so that the upper graphite surface 115b wiped off the remainder of the solution FV from the substrate. During this entire process, the cooling rate was kept at 3 ° C / minute. After the last procedural step, the tube was removed from the furnace and allowed to cool to room temperature. A substrate 31 made of η-conductive GaAs was obtained, on which four layers were epitaxially grown (FIG. 7). The first layer 32 on the substrate 31 consisted of η-conductive Ga ^ _x Al ₁ As with

509545/31

about 0.3 to 0.5 ^{doped to about 10 18} electrons / cc by Sn. An nn heterojunction 31a was therefore present at the interface between layers 31 and 32. The second layer 33 was gallium arsenide doped with silicon (and possibly with zinc by diffusion from the subsequent layer) which was compensated but p-type. A pn heterojunction 32a was formed at the interface between layers 32 and 33. The third layer 34 was p-type Ga ₁ _ _x Al “As with χ approximately from 0.3 to 0.5. This layer was p-doped by zinc to about 10 ¹⁸ to 10 ¹⁹ holes / cm ^3. A pp heterojunction 33a was present at the interface between layers 33 and 34. The fourth layer 35 was gallium arsenide, which was doped p-type ^{with germanium and about 10 18 holes / ccm.} This resulted in a further pp heterojunction 34a between layers 34 and 35.

The thicknesses of the layers 32 to 35, measured on a section, were about 5 μm, 1.5 μm, 1.9 μm or 2 to 15 / in. The distance of the pn heterojunction 32a from pp heterojunction 33a was therefore about 1.5 / im.

A laser diode without a heat sink was then fabricated from the multilayer semiconductor piece to measure the threshold current density. For this purpose, zinc with a high concentration (10 ²⁰ Zn / cm ³ ) was first diffused into the surface of the sample by surface diffusion to a depth of 0.2 μm. The substrate was then lapped to a thickness of 0.15 mm. The contacts (ie, layers 36 and 37 in FIG. 7) on the n and p surfaces were vapor-deposited in the usual manner, with layers of chromium and then of gold being deposited several thousand angstroms thick. The sample

“Ο was then cut so that several diodes were formed were arranged on holders with contacting means for the n- and p-sides of the diodes became.

These laser diodes were then placed in an infrared light microscope. The diodes were actuated with a power pulse source. At room temperature, the threshold current density of a laser diode from this sample was 3900 A / cm ² .

Other diodes made in a similar manner with an intermediate layer of narrow bandgap less than 1.0 / µm thick showed threshold values down to 3000 A / cm ² at room temperature. Finally, inside showed reflective diodes

»5 threshold values at room temperature in the range from 2300 to 2800 A / cm ² .

For this purpose 3 sheets of drawings

Claims (9)

Claims: 21 1. A method for producing a three-layer semiconductor body, the layers of which on a Gallium arsenide substrates are epitaxially grown consisting of two layers of wide energy band gap with opposite conductivity type and an intermediate layer narrower Energy band gap and which are brought into contact with one another by the Substrates with molten gallium, aluminum and arsenic containing solutions different Aluminum content and different doping and controlled cooling be deposited, characterized by the fact that the layer has a narrow band spacing by controlling their growth temperature and the cooling period during their Deposition to a thickness below two μm ao is limited. 2. The method according to claim 1, characterized in that the intermediate layer is narrow Band gap (33) by controlling their temperature and cooling period during the Growth to a thickness of less than 1.0 µm is limited. 3. The method according to claim 1 or 2, characterized in that after growing A heavily doped GaAs layer (35) is grown on the three layers for contacting purposes which has the same conductivity type as the layer immediately below it wide band gap (34). 4. The method according to any one of claims 1 to 3, characterized in that in the manufacture of a semiconductor body with an npp three- Layer sequence of the dopant in the narrow band gap for growing the intermediate layer (33) envisaged solution is sufficient to create a slightly p-type compensated intermediate layer (33) to generate. 5. The method according to any one of claims 1 to 4, characterized in that the dopant for the layer (32) adjoining the substrate (31) with a wide band gap from that of Sn, Si and Te existing group is selected. 6. The method according to claim 1 or 2, characterized in that the dopant for the intermediate layer narrow band gap (33) is selected from the group consisting of Si and Si plus Zn. ι. Method according to one of Claims 1 to 6, characterized in that the dopant for the further layer of wide band gap (34) growing on the intermediate layer (33) consists of Zn. 8. The method according to claim 3, characterized in that the dopant for the strongly do benefited Contacting layer (35) is selected from the group consisting of Zn and Ge. 9. The method according to any one of claims 1 to 8, characterized in that at least during growing the narrow bandgap intermediate layer (33) in over the substrate standing melt volume maintain an upward temperature gradient from the substrate will. 472